fix(aws-ecr-assets): correct file existence validation in tests (#19945)

In my build environment, the file existence tests in the `aws-ecr-assets` module that use `.toBeDefined()` cannot fail.  The [`fs.existsSync()`](https://nodejs.org/api/fs.html#fsexistssyncpath) function returns a boolean, and either `true` or `false` will count as defined for the `.toBeDefined()` test function.

To test the test, remove the `packages/@aws-cdk/aws-ecr-assets/test/dockerignore-image/dockerignore` file, and it should fail due to the `foobar.txt` check:
https://github.com/aws/aws-cdk/blob/4cb3b2bdb959ae398ffe2f8a5a927280f5d63306/packages/%40aws-cdk/aws-ecr-assets/test/image-asset.test.ts#L387

Fixes #19944 

----

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### Adding new Unconventional Dependencies:

* [ ] This PR adds new unconventional dependencies following the process described [here](https://github.com/aws/aws-cdk/blob/master/CONTRIBUTING.md/#adding-new-unconventional-dependencies)

### New Features

* [ ] Have you added the new feature to an [integration test](https://github.com/aws/aws-cdk/blob/master/INTEGRATION_TESTS.md)?
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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: johns1342

packages/@aws-cdk/aws-ecr-assets/test/image-asset.test.ts

@@ -258,8 +258,8 @@ describe('image asset', () => {
 
     const session = app.synth();
 
-    expect(fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, 'Dockerfile'))).toBeDefined();
-    expect(fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, 'index.py'))).toBeDefined();
+    expect(fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, 'Dockerfile'))).toBe(true);
+    expect(fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, 'index.py'))).toBe(true);
 
   });
 
@@ -288,16 +288,16 @@ describe('image asset', () => {
     const session = app.synth();
 
     // Only the files exempted above should be included.
-    expect(fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, '.dockerignore'))).toBeDefined();
-    expect(fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, 'Dockerfile'))).toBeDefined();
-    expect(fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, 'index.py'))).toBeDefined();
-    expect(fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, 'foobar.txt'))).toBeDefined();
-    expect(fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, 'subdirectory'))).toBeDefined();
-    expect(fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, 'subdirectory', 'baz.txt'))).toBeDefined();
-    expect(!fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, 'node_modules'))).toBeDefined();
-    expect(!fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, 'node_modules', 'one'))).toBeDefined();
-    expect(!fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, 'node_modules', 'some_dep'))).toBeDefined();
-    expect(!fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, 'node_modules', 'some_dep', 'file'))).toBeDefined();
+    expect(fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, '.dockerignore'))).toBe(true);
+    expect(fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, 'Dockerfile'))).toBe(true);
+    expect(fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, 'index.py'))).toBe(true);
+    expect(fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, 'foobar.txt'))).toBe(true);
+    expect(fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, 'subdirectory'))).toBe(true);
+    expect(fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, 'subdirectory', 'baz.txt'))).toBe(true);
+    expect(!fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, 'node_modules'))).toBe(true);
+    expect(!fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, 'node_modules', 'one'))).toBe(true);
+    expect(!fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, 'node_modules', 'some_dep'))).toBe(true);
+    expect(!fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, 'node_modules', 'some_dep', 'file'))).toBe(true);
   });
 
   testFutureBehavior('docker directory is staged without files specified in exclude option', flags, App, (app) => {
@@ -381,12 +381,12 @@ function testDockerDirectoryIsStagedWithoutFilesSpecifiedInDockerignore(app: App
   const session = app.synth();
 
   // .dockerignore itself should be included in output to be processed during docker build
-  expect(fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, '.dockerignore'))).toBeDefined();
-  expect(fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, 'Dockerfile'))).toBeDefined();
-  expect(fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, 'index.py'))).toBeDefined();
-  expect(!fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, 'foobar.txt'))).toBeDefined();
-  expect(fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, 'subdirectory'))).toBeDefined();
-  expect(fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, 'subdirectory', 'baz.txt'))).toBeDefined();
+  expect(fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, '.dockerignore'))).toBe(true);
+  expect(fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, 'Dockerfile'))).toBe(true);
+  expect(fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, 'index.py'))).toBe(true);
+  expect(!fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, 'foobar.txt'))).toBe(true);
+  expect(fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, 'subdirectory'))).toBe(true);
+  expect(fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, 'subdirectory', 'baz.txt'))).toBe(true);
 }
 
 function testDockerDirectoryIsStagedWithoutFilesSpecifiedInExcludeOption(app: App, ignoreMode?: IgnoreMode) {
@@ -399,12 +399,12 @@ function testDockerDirectoryIsStagedWithoutFilesSpecifiedInExcludeOption(app: Ap
 
   const session = app.synth();
 
-  expect(fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, '.dockerignore'))).toBeDefined();
-  expect(fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, 'Dockerfile'))).toBeDefined();
-  expect(fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, 'index.py'))).toBeDefined();
-  expect(!fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, 'foobar.txt'))).toBeDefined();
-  expect(!fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, 'subdirectory'))).toBeDefined();
-  expect(!fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, 'subdirectory', 'baz.txt'))).toBeDefined();
+  expect(fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, '.dockerignore'))).toBe(true);
+  expect(fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, 'Dockerfile'))).toBe(true);
+  expect(fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, 'index.py'))).toBe(true);
+  expect(!fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, 'foobar.txt'))).toBe(true);
+  expect(!fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, 'subdirectory'))).toBe(true);
+  expect(!fs.existsSync(path.join(session.directory, `asset.${image.assetHash}`, 'subdirectory', 'baz.txt'))).toBe(true);
 }
 
 testFutureBehavior('nested assemblies share assets: legacy synth edition', flags, App, (app) => {

packages/@aws-cdk/aws-ecr-assets/test/tarball-asset.test.ts

@@ -123,7 +123,7 @@ describe('image asset', () => {
 
     const session = app.synth();
 
-    expect(fs.existsSync(path.join(session.directory, `asset.${image.assetHash}.tar`))).toBeDefined();
+    expect(fs.existsSync(path.join(session.directory, `asset.${image.assetHash}.tar`))).toBe(true);
   });
 
   test('fails if the file does not exist', () => {