refactor: extracting a variable to make a test more deteministic (#23173)

There has been at least one pipeline failure due to a 1 ms difference between the actual and expected values in this test.

Extract the expiration value to reuse it for input and expectation.

----

### All Submissions:

* [ ] Have you followed the guidelines in our [Contributing guide?](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md)

### Adding new Construct Runtime Dependencies:

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

### New Features

* [ ] Have you added the new feature to an [integration test](https://github.com/aws/aws-cdk/blob/main/INTEGRATION_TESTS.md)?
	* [ ] Did you use `yarn integ` to deploy the infrastructure and generate the snapshot (i.e. `yarn integ` without `--dry-run`)?

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: otaviomacedo

packages/@aws-cdk/aws-appsync/test/appsync-auth.test.ts

@@ -90,7 +90,8 @@ describe('AppSync API Key Authorization', () => {
   });
 
   test('apiKeyConfig creates default with valid expiration date', () => {
-    const expirationDate: number = cdk.Expiration.after(cdk.Duration.days(10)).toEpoch();
+    const expires = cdk.Expiration.after(cdk.Duration.days(10));
+    const expirationDate: number = expires.toEpoch();
 
     // WHEN
     new appsync.GraphqlApi(stack, 'API', {
@@ -100,7 +101,7 @@ describe('AppSync API Key Authorization', () => {
         defaultAuthorization: {
           authorizationType: appsync.AuthorizationType.API_KEY,
           apiKeyConfig: {
-            expires: cdk.Expiration.after(cdk.Duration.days(10)),
+            expires,
           },
         },
       },