[SPARK-5074] [CORE] [TESTS] Fix the flakey test 'run shuffle with map stage failure' in DAGSchedulerSuite

Test failure: https://amplab.cs.berkeley.edu/jenkins/job/Spark-Master-SBT/AMPLAB_JENKINS_BUILD_PROFILE=hadoop2.2,label=centos/2240/testReport/junit/org.apache.spark.scheduler/DAGSchedulerSuite/run_shuffle_with_map_stage_failure/

This is because many tests share the same `JobListener`. Because after each test, `scheduler` isn't stopped. So actually it's still running. When running the test `run shuffle with map stage failure`, some previous test may trigger [ResubmitFailedStages](https://github.com/apache/spark/blob/ebc25a4ddfe07a67668217cec59893bc3b8cf730/core/src/main/scala/org/apache/spark/scheduler/DAGScheduler.scala#L1120) logic, and report `jobFailed` and override the global `failure` variable.

This PR uses `after` to call `scheduler.stop()` for each test.

Author: zsxwing <zsxwing@gmail.com>

Closes #5903 from zsxwing/SPARK-5074 and squashes the following commits:

1e6f13e [zsxwing] Fix the flakey test 'run shuffle with map stage failure' in DAGSchedulerSuite

Author: zsxwing

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

@@ -174,6 +174,10 @@ class DAGSchedulerSuite
     dagEventProcessLoopTester = new DAGSchedulerEventProcessLoopTester(scheduler)
   }
 
+  after {
+    scheduler.stop()
+  }
+
   override def afterAll() {
     super.afterAll()
   }
@@ -261,8 +265,9 @@ class DAGSchedulerSuite
       override def taskSucceeded(partition: Int, value: Any) = numResults += 1
       override def jobFailed(exception: Exception) = throw exception
     }
-    submit(new MyRDD(sc, 0, Nil), Array(), listener = fakeListener)
+    val jobId = submit(new MyRDD(sc, 0, Nil), Array(), listener = fakeListener)
     assert(numResults === 0)
+    cancel(jobId)
   }
 
   test("run trivial job") {