Unit test to ensure retransmitted message reaches the peer exchange

[pan-apple]

Problem

Need an end to end test where

1. A peer registers a unsolicited message handler
2. Message sent by the sender gets dropped
3. The message is retransmitted via CRMP after ack timeout
4. The peer receives the message via handler
5. The sender get the ack for the message
6. All the state is cleaned up correctly

Change overview

Added the test mentioned in the problem statement.

Testing

CheckResendApplicationMessageWithPeerExchange is the new test.
Only unit test was added, and no functional code was modified.

[pan-apple]

rebased