Let the test stop gracefully and don't throw exception.

Author: viirya

streaming/src/test/scala/org/apache/spark/streaming/StreamingContextSuite.scala

@@ -434,7 +434,7 @@ package object testPackage extends Assertions {
         assert(rddGenerated && foreachCallSiteCorrect, "Call site in foreachRDD was not correct")
       }
     } finally {
-      ssc.stop()
+      ssc.stop(stopSparkContext = false, stopGracefully = true)
     }
   }
 }