Don't create SparkContext in JobProgressListenerSuite.

This reduces the time of the test from 11 seconds to 20 milliseconds.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #324 from pwendell/job-test and squashes the following commits:

868d9eb [Patrick Wendell] Don't create SparkContext in JobProgressListenerSuite.

core/src/test/scala/org/apache/spark/ui/jobs/JobProgressListenerSuite.scala

@@ -55,8 +55,8 @@ class JobProgressListenerSuite extends FunSuite with LocalSparkContext with Shou
   }
 
   test("test executor id to summary") {
-    val sc = new SparkContext("local", "test")
-    val listener = new JobProgressListener(sc.conf)
+    val conf = new SparkConf()
+    val listener = new JobProgressListener(conf)
     val taskMetrics = new TaskMetrics()
     val shuffleReadMetrics = new ShuffleReadMetrics()