JobMasterServiceLeadershipRunnerTest#testResultFutureCompletionOfOutdatedLeaderIsIgnored isn't implemented properly

testResultFutureCompletionOfOutdatedLeaderIsIgnored doesn't test the desired behavior: The TestingJobMasterService#closeAsync() callback throws an UnsupportedOperationException by default which prevents the test from properly finalizing the leadership revocation.

The test is still passing because the test checks implicitly for this error. Instead, we should verify that the runner's resultFuture doesn't complete until the runner is closed.