chore(events-targets): migrate a test to use assertions' annotations class (#19465)

Also removes unnecessary assertion on number of messages, as this breaks when other messages are introduced. See #19464, which introduces a relevant message and breaks this test. Fixing in one other place as well. 

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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: kaizencc

packages/@aws-cdk/aws-events-targets/test/aws-api/aws-api.test.ts

@@ -1,7 +1,7 @@
-import { Template } from '@aws-cdk/assertions';
+import { Annotations, Template } from '@aws-cdk/assertions';
 import * as events from '@aws-cdk/aws-events';
 import * as iam from '@aws-cdk/aws-iam';
-import { App, Stack } from '@aws-cdk/core';
+import { Stack } from '@aws-cdk/core';
 import * as targets from '../../lib';
 
 test('use AwsApi as an event rule target', () => {
@@ -163,9 +163,5 @@ test('with service not in AWS SDK', () => {
   rule.addTarget(awsApi);
 
   // THEN
-  const assembly = App.of(stack)!.synth().getStackArtifact(stack.stackName);
-  expect(assembly.messages.length).toBe(1);
-  const message = assembly.messages[0];
-  expect(message.entry.type).toBe('aws:cdk:warning');
-  expect(message.entry.data).toBe('Service no-such-service does not exist in the AWS SDK. Check the list of available services and actions from https://docs.aws.amazon.com/AWSJavaScriptSDK/latest/index.html');
+  Annotations.fromStack(stack).hasWarning('*', 'Service no-such-service does not exist in the AWS SDK. Check the list of available services and actions from https://docs.aws.amazon.com/AWSJavaScriptSDK/latest/index.html');
 });