fix(cli-integ): hardcoded qualifier and toolkit stack name in cdk diff with large changeset... test (#79)

When these are hardcoded, it makes the test less stable in case of
potential cleanup failures. For example, canary runs now exhibit this
error:

```console

ailed resources:
--
751 | custom-stack2 \| 4:57:43 AM \| CREATE_FAILED        \| AWS::S3::Bucket            \| StagingBucket Resource handler returned message: "cdk-abc1111-assets-222461455105-us-east-1 already exists (Service: S3, Status Code: 0, Request ID: null)" (RequestToken: 98d6b74c-97d6-2b80-9e9f-a4d4eb550e2b, HandlerErrorCode: AlreadyExists)
752 | ❌  Environment aws://222461455105/us-east-1 failed bootstrapping: _ToolkitError: The stack named custom-stack2 failed creation, it may need to be manually deleted from the AWS console: ROLLBACK_COMPLETE: Resource handler returned message: "cdk-abc1111-assets-222461455105-us-east-1 already exists (Service: S3, Status Code: 0, Request ID: null)" (RequestToken: 98d6b74c-97d6-2b80-9e9f-a4d4eb550e2b, HandlerErrorCode: AlreadyExists)
753 | at FullCloudFormationDeployment.monitorDeployment (/tmp/tmpcdkXE8YYD/node_modules/aws-cdk/lib/index.js:140591:17)
754 | at process.processTicksAndRejections (node:internal/process/task_queues:95:5)
755 | at async _BootstrapStack.update (/tmp/tmpcdkXE8YYD/node_modules/aws-cdk/lib/index.js:172482:21)
756 | at async /tmp/tmpcdkXE8YYD/node_modules/aws-cdk/lib/index.js:333085:29 {
757 | type: 'toolkit',
758 | source: 'toolkit',
759 | cause: undefined
760 | }
```

I cleaned up the environment, but lets also make the test create a
unique stack every time it runs.

Author: iliapolo

packages/@aws-cdk-testing/cli-integ/tests/cli-integ-tests/cdk-cdk-diff-with-large-changeset-and-custom-toolkit-stack-name-and-qualifier-does-not-fail.integtest.ts

@@ -4,8 +4,9 @@ jest.setTimeout(2 * 60 * 60_000); // Includes the time to acquire locks, worst-c
 
 integTest('cdk diff with large changeset and custom toolkit stack name and qualifier does not fail', withoutBootstrap(async (fixture) => {
   // Bootstrapping with custom toolkit stack name and qualifier
-  const qualifier = 'abc1111';
-  const toolkitStackName = 'custom-stack2';
+  const qualifier = fixture.qualifier;
+
+  const toolkitStackName = fixture.bootstrapStackName;
   await fixture.cdkBootstrapModern({
     verbose: true,
     toolkitStackName: toolkitStackName,