ISSUE-2647: Flaky Test: DeadLetterTopicTest.testDeadLetterTopicWithMultiTopic

[sijie]

Original Issue: apache#2647

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It has been seen multiple times in CI.

Method org.apache.pulsar.client.api.DeadLetterTopicTest.testDeadLetterTopicWithMultiTopic() didn't finish within the time-out 120000
Stacktrace
org.testng.internal.thread.ThreadTimeoutException: Method org.apache.pulsar.client.api.DeadLetterTopicTest.testDeadLetterTopicWithMultiTopic() didn't finish within the time-out 120000
	at java.util.concurrent.FutureTask.run(FutureTask.java:271)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at java.lang.Thread.run(Thread.java:748)