[SPARK-24663][STREAMING][TESTS] StreamingContextSuite: Wait until slow receiver has been initialized, but with hard timeout

### What changes were proposed in this pull request?

This patch fixes the flaky test failure from StreamingContextSuite "stop slow receiver gracefully", via putting flag whether initializing slow receiver is completed, and wait for such flag to be true. As receiver should be submitted via job and initialized in executor, 500ms might not be enough for covering all cases.

### Why are the changes needed?

We got some reports for test failure on this test. Please refer [SPARK-24663](https://issues.apache.org/jira/browse/SPARK-24663)

### Does this PR introduce any user-facing change?

No

### How was this patch tested?

Modified UT. I've artificially made delay on handling job submission via adding below code in `DAGScheduler.submitJob`:

```
if (rdd != null && rdd.name != null && rdd.name.startsWith("Receiver")) {
  println(s"Receiver Job! rdd name: ${rdd.name}")
  Thread.sleep(1000)
}
```

and the test "stop slow receiver gracefully" failed on current master and passed on the patch.

Closes apache#25725 from HeartSaVioR/SPARK-24663.

Authored-by: Jungtaek Lim (HeartSaVioR) <kabhwan@gmail.com>
Signed-off-by: Marcelo Vanzin <vanzin@cloudera.com>

Author: HeartSaVioR

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

@@ -346,7 +346,6 @@ class StreamingContextSuite
     logInfo("==================================\n\n\n")
     ssc = new StreamingContext(sc, Milliseconds(100))
     var runningCount = 0
-    SlowTestReceiver.receivedAllRecords = false
     // Create test receiver that sleeps in onStop()
     val totalNumRecords = 15
     val recordsPerSecond = 1
@@ -358,6 +357,9 @@ class StreamingContextSuite
     }
     ssc.start()
     ssc.awaitTerminationOrTimeout(500)
+    eventually(timeout(10.seconds), interval(10.millis)) {
+      assert(SlowTestReceiver.initialized)
+    }
     ssc.stop(stopSparkContext = false, stopGracefully = true)
     logInfo("Running count = " + runningCount)
     assert(runningCount > 0)
@@ -949,6 +951,7 @@ class SlowTestReceiver(totalRecords: Int, recordsPerSecond: Int)
   extends Receiver[Int](StorageLevel.MEMORY_ONLY) with Logging {
 
   var receivingThreadOption: Option[Thread] = None
+  @volatile var receivedAllRecords = false
 
   def onStart() {
     val thread = new Thread() {
@@ -958,25 +961,26 @@ class SlowTestReceiver(totalRecords: Int, recordsPerSecond: Int)
           Thread.sleep(1000 / recordsPerSecond)
           store(i)
         }
-        SlowTestReceiver.receivedAllRecords = true
+        receivedAllRecords = true
         logInfo(s"Received all $totalRecords records")
       }
     }
     receivingThreadOption = Some(thread)
     thread.start()
+    SlowTestReceiver.initialized = true
   }
 
   def onStop() {
     // Simulate slow receiver by waiting for all records to be produced
-    while (!SlowTestReceiver.receivedAllRecords) {
+    while (!receivedAllRecords) {
       Thread.sleep(100)
     }
     // no clean to be done, the receiving thread should stop on it own
   }
 }
 
 object SlowTestReceiver {
-  var receivedAllRecords = false
+  var initialized = false
 }
 
 /** Streaming application for testing DStream and RDD creation sites */