[SPARK-10431] [CORE] Fix intermittent test failure. Wait for event queue to be clear

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

Closes #8582 from robbinspg/InputOutputMetricsSuite.

core/src/test/scala/org/apache/spark/metrics/InputOutputMetricsSuite.scala

@@ -286,6 +286,10 @@ class InputOutputMetricsSuite extends SparkFunSuite with SharedSparkContext
   private def runAndReturnMetrics(job: => Unit,
       collector: (SparkListenerTaskEnd) => Option[Long]): Long = {
     val taskMetrics = new ArrayBuffer[Long]()
+
+    // Avoid receiving earlier taskEnd events
+    sc.listenerBus.waitUntilEmpty(500)
+
     sc.addSparkListener(new SparkListener() {
       override def onTaskEnd(taskEnd: SparkListenerTaskEnd) {
         collector(taskEnd).foreach(taskMetrics += _)