chore: delete integration stacks serially

This is a follow-up to #14505 which in retrospect wasn't complete.

The list of stacks was ordered properly, but since all stacks were being
deleted in parallel this didn't actually solve the race condition.

Delete stacks in series instead.

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

@@ -45,6 +45,8 @@ export class AwsClients {
   public async deleteStacks(...stackNames: string[]) {
     if (stackNames.length === 0) { return; }
 
+    // We purposely do all stacks serially, because they've been ordered
+    // to do the bootstrap stack last.
     for (const stackName of stackNames) {
       await this.cloudFormation('updateTerminationProtection', {
         EnableTerminationProtection: false,
@@ -53,19 +55,17 @@ export class AwsClients {
       await this.cloudFormation('deleteStack', {
         StackName: stackName,
       });
-    }
 
-    await retry(this.output, `Deleting ${stackNames}`, retry.forSeconds(600), async () => {
-      for (const stackName of stackNames) {
+      await retry(this.output, `Deleting ${stackName}`, retry.forSeconds(600), async () => {
         const status = await this.stackStatus(stackName);
         if (status !== undefined && status.endsWith('_FAILED')) {
           throw retry.abort(new Error(`'${stackName}' is in state '${status}'`));
         }
         if (status !== undefined) {
           throw new Error(`Delete of '${stackName}' not complete yet`);
         }
-      }
-    });
+      });
+    }
   }
 
   public async stackStatus(stackName: string): Promise<string | undefined> {