[SPARK-7835] Refactor HeartbeatReceiverSuite for coverage + cleanup

The existing test suite has a lot of duplicate code and doesn't even cover the most fundamental feature of the HeartbeatReceiver, which is expiring hosts that have not responded in a while.

This introduces manual clocks in `HeartbeatReceiver` and makes it respond to heartbeats only for registered executors. A few internal messages are moved to `receiveAndReply` to increase determinism of the tests so we don't have to rely on flaky constructs like `eventually`.

Author: Andrew Or <andrew@databricks.com>

Closes #7173 from andrewor14/heartbeat-receiver-tests and squashes the following commits:

4a903d6 [Andrew Or] Increase HeartReceiverSuite coverage and clean up