@Test
  public void testUnsubscribeNonExistentListeners() {
    // unsubscribe a non-existent listener should be silent
    helixNotifier.unsubscribe(refTopics.get(0), listeners.get(0));
  }
Tests unsubscribing a non-existent {@link TopicListener}. This should not throw any exception.
ambry-commons/src/test/java/com/github/ambry/commons/HelixNotifierTest.java