[SPARK-6700] [yarn] Re-enable flaky test.

Test runs have been successful on jenkins. So let's re-enable the test and look out for any failures, and fix things appropriately.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #5459 from vanzin/SPARK-6700 and squashes the following commits:

2ead85b [Marcelo Vanzin] WIP: re-enable flaky test to catch failure in jenkins.

yarn/src/test/scala/org/apache/spark/deploy/yarn/YarnClusterSuite.scala

@@ -144,7 +144,7 @@ class YarnClusterSuite extends FunSuite with BeforeAndAfterAll with Matchers wit
   }
 
   // Enable this once fix SPARK-6700
-  ignore("run Python application in yarn-cluster mode") {
+  test("run Python application in yarn-cluster mode") {
     val primaryPyFile = new File(tempDir, "test.py")
     Files.write(TEST_PYFILE, primaryPyFile, UTF_8)
     val pyFile = new File(tempDir, "test2.py")