chore: fix new ecs test to use v2 asset style (#18849)

The test "expose image name" makes an assertion about the asset image location, which differs between v1 and v2. Use `testFutureBehavior` to force it to use the v2 asset style.

This unblocks the forward merge between v1 and v2 (#18763).


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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: Elad Ben-Israel

packages/@aws-cdk/aws-ecs/test/container-definition.test.ts

@@ -2006,9 +2006,9 @@ describe('container definition', () => {
 
   });
 
-  test('exposes image name', () => {
+  testFutureBehavior('exposes image name', { '@aws-cdk/core:newStyleStackSynthesis': true }, cdk.App, (app) => {
     // GIVEN
-    const stack = new cdk.Stack();
+    const stack = new cdk.Stack(app, 'MyStack');
     const taskDefinition = new ecs.FargateTaskDefinition(stack, 'TaskDef');
 
     // WHEN
@@ -2018,17 +2018,7 @@ describe('container definition', () => {
 
     // THEN
     expect(stack.resolve(container.imageName)).toEqual({
-      'Fn::Join': [
-        '',
-        [
-          { Ref: 'AWS::AccountId' },
-          '.dkr.ecr.',
-          { Ref: 'AWS::Region' },
-          '.',
-          { Ref: 'AWS::URLSuffix' },
-          '/aws-cdk/assets:baa2d6eb2a17c75424df631c8c70ff39f2d5f3bee8b9e1a109ee24ca17300540',
-        ],
-      ],
+      'Fn::Sub': '${AWS::AccountId}.dkr.ecr.${AWS::Region}.${AWS::URLSuffix}/cdk-hnb659fds-container-assets-${AWS::AccountId}-${AWS::Region}:baa2d6eb2a17c75424df631c8c70ff39f2d5f3bee8b9e1a109ee24ca17300540',
     });
   });
 });