[SPARK-13399][STREAMING] Fix checkpointsuite type erasure warnings

## What changes were proposed in this pull request?

Change the checkpointsuite getting the outputstreams to explicitly be unchecked on the generic type so as to avoid the warnings. This only impacts test code.

Alternatively we could encode the type tag in the TestOutputStreamWithPartitions and filter the type tag as well - but this is unnecessary since multiple testoutputstreams are not registered and the previous code was not actually checking this type.

## How was the this patch tested?

unit tests (streaming/testOnly org.apache.spark.streaming.CheckpointSuite)

Author: Holden Karau <holden@us.ibm.com>

Closes #11286 from holdenk/SPARK-13399-checkpointsuite-type-erasure.

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

@@ -133,6 +133,17 @@ trait DStreamCheckpointTester { self: SparkFunSuite =>
     new StreamingContext(SparkContext.getOrCreate(conf), batchDuration)
   }
 
+  /**
+   * Get the first TestOutputStreamWithPartitions, does not check the provided generic type.
+   */
+  protected def getTestOutputStream[V: ClassTag](streams: Array[DStream[_]]):
+    TestOutputStreamWithPartitions[V] = {
+    streams.collect {
+      case ds: TestOutputStreamWithPartitions[V @unchecked] => ds
+    }.head
+  }
+
+
   protected def generateOutput[V: ClassTag](
       ssc: StreamingContext,
       targetBatchTime: Time,
@@ -150,9 +161,7 @@ trait DStreamCheckpointTester { self: SparkFunSuite =>
       clock.setTime(targetBatchTime.milliseconds)
       logInfo("Manual clock after advancing = " + clock.getTimeMillis())
 
-      val outputStream = ssc.graph.getOutputStreams().filter { dstream =>
-        dstream.isInstanceOf[TestOutputStreamWithPartitions[V]]
-      }.head.asInstanceOf[TestOutputStreamWithPartitions[V]]
+      val outputStream = getTestOutputStream[V](ssc.graph.getOutputStreams())
 
       eventually(timeout(10 seconds)) {
         ssc.awaitTerminationOrTimeout(10)
@@ -908,9 +917,7 @@ class CheckpointSuite extends TestSuiteBase with DStreamCheckpointTester
     logInfo("Manual clock after advancing = " + clock.getTimeMillis())
     Thread.sleep(batchDuration.milliseconds)
 
-    val outputStream = ssc.graph.getOutputStreams().filter { dstream =>
-      dstream.isInstanceOf[TestOutputStreamWithPartitions[V]]
-    }.head.asInstanceOf[TestOutputStreamWithPartitions[V]]
+    val outputStream = getTestOutputStream[V](ssc.graph.getOutputStreams())
     outputStream.output.asScala.map(_.flatten)
   }
 }