[Spark-753] Fix ClusterSchedulSuite unit test failed 

Due to some non-determinism scenarios in unit test, it will failed depend on environment, for examples

info ClusterSchedulerSuite:
info - FIFO Scheduler Test
INFO 05/30/2013 14:56:15.332 spray-io-worker-0 IoWorker IoWorker thread 'spray-io-worker-0' stopped info - Fair Scheduler Test *** FAILED ***
info 3 did not equal 0 (ClusterSchedulerSuite.scala:111)
INFO 05/30/2013 14:56:15.540 spray-io-worker-0 IoWorker IoWorker thread 'spray-io-worker-0' stopped info - Nested Pool Test *** FAILED ***
info 7 did not equal 0 (ClusterSchedulerSuite.scala:111)
INFO 05/30/2013 14:56:15.712 spray-io-worker-0 IoWorker IoWorker thread 'spray-io-worker-0' stopped error Failed: : Total 3, Failed 2, Errors 0, Passed 1, Skipped 0 error Failed tests:
error spark.scheduler.ClusterSchedulerSuite
error (core/test:test-only) sbt.TestsFailedException: Tests unsuccessful error Total time: 5 s, completed May 30, 2013 2:56:15 PM