chore(core): verify stack artifacts are written in topological order (#8515)

Add a test to verify stacks are serialized to the manifest file in a topological order, based on their dependencies. By adding this test we are committing to maintain the topological order of the stacks in the manifest file, allowing users to consume it without needing to sort the stacks. 

Note that all of the CDK toolkit commands such as `cdk list` and `cdk deploy` already assume this and do not sort stacks upon execution.
> see [`cdk list`](https://github.com/aws/aws-cdk/blob/master/packages/aws-cdk/lib/cdk-toolkit.ts#L264) and [`cdk deploy`](https://github.com/aws/aws-cdk/blob/master/packages/aws-cdk/lib/cdk-toolkit.ts#L111) for reference  

I have initially added the test in `cx-api` module but after more consideration I think it is better suited in the `core` module since we want to verify this behavior in the context of a CDK application. I have left both tests in this PR for the sake of discussion in the event the reviewer thinks this actually fits better in `cx-api`.

helps #8436

----

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

Author: NetaNir

Diff for: packages/@aws-cdk/core/test/test.app.ts

@@ -325,6 +325,32 @@ export = {
 
     test.done();
   },
+
+  'stacks are written to the assembly file in a topological order'(test: Test) {
+    // WHEN
+    const assembly = withApp({}, (app) => {
+      const stackC = new Stack(app, 'StackC');
+      const stackD = new Stack(app, 'StackD');
+      const stackA = new Stack(app, 'StackA');
+      const stackB = new Stack(app, 'StackB');
+
+      // Create the following dependency order:
+      // A ->
+      //      C -> D
+      // B ->
+      stackC.addDependency(stackA);
+      stackC.addDependency(stackB);
+      stackD.addDependency(stackC);
+    });
+
+    // THEN
+    const artifactsIds = assembly.artifacts.map(a => a.id);
+    test.ok(artifactsIds.indexOf('StackA') < artifactsIds.indexOf('StackC'));
+    test.ok(artifactsIds.indexOf('StackB') < artifactsIds.indexOf('StackC'));
+    test.ok(artifactsIds.indexOf('StackC') < artifactsIds.indexOf('StackD'));
+
+    test.done();
+  },
 };
 
 class MyConstruct extends Construct {

Diff for: packages/@aws-cdk/cx-api/test/cloud-assembly-builder.test.ts

@@ -166,4 +166,62 @@ test('write and read nested cloud assembly artifact', () => {
 
   const nested = art?.nestedAssembly;
   expect(nested?.artifacts.length).toEqual(0);
+});
+
+test('artifcats are written in topological order', () => {
+  // GIVEN
+  const outdir = fs.mkdtempSync(path.join(os.tmpdir(), 'cloud-assembly-builder-tests'));
+  const session = new cxapi.CloudAssemblyBuilder(outdir);
+  const templateFile = 'foo.template.json';
+
+  const innerAsmDir = path.join(outdir, 'hello');
+  new cxapi.CloudAssemblyBuilder(innerAsmDir).buildAssembly();
+
+  // WHEN
+
+  // Create the following dependency order:
+  // A ->
+  //      C -> D
+  // B ->
+  session.addArtifact('artifact-D', {
+    type: cxschema.ArtifactType.AWS_CLOUDFORMATION_STACK,
+    environment: 'aws://1222344/us-east-1',
+    dependencies: ['artifact-C'],
+    properties: {
+      templateFile,
+    },
+  });
+
+  session.addArtifact('artifact-C', {
+    type: cxschema.ArtifactType.AWS_CLOUDFORMATION_STACK,
+    environment: 'aws://1222344/us-east-1',
+    dependencies: ['artifact-B', 'artifact-A'],
+    properties: {
+      templateFile,
+    },
+  });
+
+  session.addArtifact('artifact-B', {
+    type: cxschema.ArtifactType.AWS_CLOUDFORMATION_STACK,
+    environment: 'aws://1222344/us-east-1',
+    properties: {
+      templateFile,
+    },
+  });
+
+  session.addArtifact('artifact-A', {
+    type: cxschema.ArtifactType.AWS_CLOUDFORMATION_STACK,
+    environment: 'aws://1222344/us-east-1',
+    properties: {
+      templateFile,
+    },
+  });
+
+  const asm = session.buildAssembly();
+  const artifactsIds = asm.artifacts.map(a => a.id);
+
+  // THEN
+  expect(artifactsIds.indexOf('artifact-A')).toBeLessThan(artifactsIds.indexOf('artifact-C'));
+  expect(artifactsIds.indexOf('artifact-B')).toBeLessThan(artifactsIds.indexOf('artifact-C'));
+  expect(artifactsIds.indexOf('artifact-C')).toBeLessThan(artifactsIds.indexOf('artifact-D'));
 });

Diff for: packages/@aws-cdk/cx-api/test/cloud-assembly.test.ts

@@ -85,7 +85,7 @@ test('assets', () => {
 test('can-read-0.36.0', () => {
   // WHEN
   new CloudAssembly(path.join(FIXTURES, 'single-stack-0.36'));
-  // THEN: no eexception
+  // THEN: no exception
   expect(true).toBeTruthy();
 });