Add receiver scheduling policy test for Streaming

[zsxwing]

Added tests for the new receiver scheduling policy in 1.5. This PR is for SPARK-9933: Test the new receiver scheduling.

• ReceiverLoadBalanceTest: test if receivers are load balanced in the beginning by running the app many times and verifying whether they are load balanced. Failed when using branch-1.5, Passed when using https://github.com/apache/spark/pull/8340
• ReceiverFailureTest: test whether receivers are restarted after any failure, independent of the max task attempts. Passed
• ReceiverTimeoutTest: test a timeout after which if all the receivers have not started, it should fail. Failed when using branch-1.5, Passed when using https://github.com/apache/spark/pull/8242

[zsxwing]

Added a commit to improve the test coverage. This commit helped me find the issue SPARK-10137.