MINOR: mark flaky test

[lianetm]

Pretty flaky on trunk lately, showing up on unrelated open PRs Tracked
to fix with https://issues.apache.org/jira/browse/KAFKA-20345

[lianetm]

cc. @squah-confluent just for awareness (or in case you may have thoughts to share on the jira on how to fix). Thanks!

[chia7712]

The root cause is that the completion of the Runnable does not mean recordBackgroundProcessingTime has finished. We should add verify(metrics, timeout(5000)).recordBackgroundProcessingTime to ensure the metric is actually recorded

            // Task 1 takes 100 ms.
            mockTime.sleep(100);
            task1Unblocked.countDown();
            task1.get(5, TimeUnit.SECONDS);
            verify(metrics, timeout(5000)).recordBackgroundProcessingTime(100);

            // Task 3 starts.
            task3Started.await();

            // Task 2 takes 500 ms.
            mockTime.sleep(400);
            task2Unblocked.countDown();
            task2.get(5, TimeUnit.SECONDS);
            verify(metrics, timeout(5000)).recordBackgroundProcessingTime(500);

            // Task 3 takes 500 ms.
            mockTime.sleep(100);
            task3Unblocked.countDown();
            task3.get(5, TimeUnit.SECONDS);
            verify(metrics, timeout(5000).times(2)).recordBackgroundProcessingTime(500);