TestZKFailoverController#testGracefulFailoverMultipleZKfcs uses fixed time sleep before assertions. This may fail sometimes though 10 seconds are generally long enough. I think we can use GenericTestUtils.waitFor() to retry the assertions. Meanwhile, it usually does not need 10 seconds to reach the condition (<1s in my laptop). Removing the 10s sleep will also make the UT run faster.
This is also true to TestZKFailoverController#testGracefulFailover.