[SPARK-18846][SCHEDULER] Fix flakiness in SchedulerIntegrationSuite

There is a small race in SchedulerIntegrationSuite.
The test assumes that the taskscheduler thread
processing that last task will finish before the DAGScheduler processes
the task event and notifies the job waiter, but that is not 100%
guaranteed.

ran the test locally a bunch of times, never failed, though admittedly
it never failed locally for me before either.  However I am nearly 100%
certain this is what caused the failure of one jenkins build
https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder/68694/consoleFull
(which is long gone now, sorry -- I fixed it as part of
https://github.com/apache/spark/pull/14079 initially)

Author: Imran Rashid <irashid@cloudera.com>

Closes #16270 from squito/sched_integ_flakiness.