[SPARK-10058] [CORE] [TESTS] Fix the flaky tests in HeartbeatReceiverSuite

Fixed the test failure here: https://amplab.cs.berkeley.edu/jenkins/view/Spark-QA-Test/job/Spark-1.5-SBT/116/AMPLAB_JENKINS_BUILD_PROFILE=hadoop2.2,label=spark-test/testReport/junit/org.apache.spark/HeartbeatReceiverSuite/normal_heartbeat/

This failure is because `HeartbeatReceiverSuite. heartbeatReceiver` may receive `SparkListenerExecutorAdded("driver")` sent from [LocalBackend](https://github.com/apache/spark/blob/8fb3a65cbb714120d612e58ef9d12b0521a83260/core/src/main/scala/org/apache/spark/scheduler/local/LocalBackend.scala#L121).

There are other race conditions in `HeartbeatReceiverSuite` because `HeartbeatReceiver.onExecutorAdded` and `HeartbeatReceiver.onExecutorRemoved` are asynchronous. This PR also fixed them.

Author: zsxwing <zsxwing@gmail.com>

Closes #8946 from zsxwing/SPARK-10058.