chore: bootstrap stack too old for integ tests (#17277)

The integ tests try to be clever to save time, and rebootstrap an
account and region pair only if the bootstrap stack does not exist
yet.

This is not good enough if the **version** of the bootstrap stack
changes though (no rebootstrapping will happen), and the following
error will occur:

```
❌  cdktest-0n94n0po827f-test-2 failed: Error: cdktest-0n94n0po827f-test-2: This CDK deployment requires bootstrap stack version '6', found '4'. Please run 'cdk bootstrap'.
```

Instead, always bootstrap every account/region pair at least once
per run. It will take some time, but in most cases we'll be able to
short-circuit the CFN deployment, so it will take ~2s instead of
~20 per case.


----

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

packages/aws-cdk/test/integ/helpers/cdk.ts

@@ -541,11 +541,12 @@ let sanityChecked: boolean | undefined;
  * by hand so let's just mass-automate it.
  */
 async function ensureBootstrapped(fixture: TestFixture) {
-  // Use the default name for the bootstrap stack
-  if (await fixture.aws.stackStatus('CDKToolkit') === undefined) {
-    // use whatever version of bootstrap is the default for this particular version of the CLI
-    await fixture.cdk(['bootstrap', `aws://${await fixture.aws.account()}/${fixture.aws.region}`]);
-  }
+  // use whatever version of bootstrap is the default for this particular version of the CLI
+  const envSpecifier = `aws://${await fixture.aws.account()}/${fixture.aws.region}`;
+  if (ALREADY_BOOTSTRAPPED_IN_THIS_RUN.has(envSpecifier)) { return; }
+
+  await fixture.cdk(['bootstrap', envSpecifier]);
+  ALREADY_BOOTSTRAPPED_IN_THIS_RUN.add(envSpecifier);
 }
 
 /**
@@ -689,3 +690,5 @@ const installNpm7 = memoize0(async (): Promise<string> => {
 
   return path.join(installDir, 'node_modules', '.bin', 'npm');
 });
+
+const ALREADY_BOOTSTRAPPED_IN_THIS_RUN = new Set();