gapic-showcase: ITLongRunningOperation flaky tests

[zhumin8]

Observed this failure several times in last couple of days.

Error:  Failures: 
Error:    ITLongRunningOperation.testHttpJson_LROUnsuccessfulResponse_exceedsTotalTimeout_throwsDeadlineExceededException:187 Unexpected exception type thrown, expected: <java.util.concurrent.CancellationException> but was: <java.util.concurrent.ExecutionException>

link to full log.