[SPARK-9504] [STREAMING] [TESTS] Use eventually to fix the flaky test

The previous code uses `ssc.awaitTerminationOrTimeout(500)`. Since nobody will stop it during `awaitTerminationOrTimeout`, it's just like `sleep(500)`. In a super overloaded Jenkins worker, the receiver may be not able to start in 500 milliseconds. Verified this in the log of https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder/39149/ There is no log about starting the receiver before this failure. That's why `assert(runningCount > 0)` failed.

This PR replaces `awaitTerminationOrTimeout` with `eventually` which should be more reliable.

Author: zsxwing <zsxwing@gmail.com>

Closes #7823 from zsxwing/SPARK-9504 and squashes the following commits:

7af66a6 [zsxwing] Remove wrong assertion
5ba2c99 [zsxwing] Use eventually to fix the flaky test

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

@@ -261,7 +261,7 @@ class StreamingContextSuite extends SparkFunSuite with BeforeAndAfter with Timeo
     for (i <- 1 to 4) {
       logInfo("==================================\n\n\n")
       ssc = new StreamingContext(sc, Milliseconds(100))
-      var runningCount = 0
+      @volatile var runningCount = 0
       TestReceiver.counter.set(1)
       val input = ssc.receiverStream(new TestReceiver)
       input.count().foreachRDD { rdd =>
@@ -270,14 +270,14 @@ class StreamingContextSuite extends SparkFunSuite with BeforeAndAfter with Timeo
         logInfo("Count = " + count + ", Running count = " + runningCount)
       }
       ssc.start()
-      ssc.awaitTerminationOrTimeout(500)
+      eventually(timeout(10.seconds), interval(10.millis)) {
+        assert(runningCount > 0)
+      }
       ssc.stop(stopSparkContext = false, stopGracefully = true)
       logInfo("Running count = " + runningCount)
       logInfo("TestReceiver.counter = " + TestReceiver.counter.get())
-      assert(runningCount > 0)
       assert(
-        (TestReceiver.counter.get() == runningCount + 1) ||
-          (TestReceiver.counter.get() == runningCount + 2),
+        TestReceiver.counter.get() == runningCount + 1,
         "Received records = " + TestReceiver.counter.get() + ", " +
           "processed records = " + runningCount
       )