test_misc.py: shutdown the heartbeat thread so it doesn't go into a busy loop
test_misc.py: shutdown the heartbeat thread so it doesn't go into a busy loop

HeartbeatListener relies on the transport to call its on_send() method to update
received_heartbeat, so it knows how long to wait before sending the next heartbeat.
test_heartbeatlistener() mocks the transport, resulting in the callback never happening.
When running the full test suite, after receive_sleep has elapsed, the heartbeat thread
goes into a busy loop sending heartbeat frames and spamming the log. Explicitly shut down
the heartbeat thread using HeartbeatListener.on_disconnected() to avoid this.

jasonrbriggs/stomp.py,jasonrbriggs/stomp.py