[SPARK-10454] [SPARK CORE] wait for empty event queue

Author: robbins <robbins@uk.ibm.com>

Closes #8605 from robbinspg/DAGSchedulerSuite-fix.

core/src/test/scala/org/apache/spark/scheduler/DAGSchedulerSuite.scala

@@ -812,6 +812,7 @@ class DAGSchedulerSuite
     }
 
     // The map stage should have been submitted.
+    sc.listenerBus.waitUntilEmpty(WAIT_TIMEOUT_MILLIS)
     assert(countSubmittedMapStageAttempts() === 1)
 
     complete(taskSets(0), Seq(