[SPARK-9504] [STREAMING] [TESTS] Fix o.a.s.streaming.StreamingContextSuite.stop gracefully again

The test failure is here: https://amplab.cs.berkeley.edu/jenkins/job/Spark-Master-SBT/3150/AMPLAB_JENKINS_BUILD_PROFILE=hadoop1.0,label=centos/testReport/junit/org.apache.spark.streaming/StreamingContextSuite/stop_gracefully/

There is a race condition in TestReceiver that it may add 1 record and increase `TestReceiver.counter` after stopping `BlockGenerator`. This PR just adds `join` to wait the pushing thread.

Author: zsxwing <zsxwing@gmail.com>

Closes #7934 from zsxwing/SPARK-9504-2 and squashes the following commits:

cfd7973 [zsxwing] Wait for the thread to make sure we won't change TestReceiver.counter after stopping BlockGenerator