TMSS-320: clear running flag so thread loops can exit

LCS/Messaging/python/messaging/messagebus.py

@@ -1456,6 +1456,7 @@ class BusListener:
             # check if the _listen_loop was started successfully
             logger.debug("waiting for thread %s to be running...", thread_name)
             if not (thread_started_event.wait(timeout=10) and thread.is_alive()):
+                self._running.clear() # clear running flag so thread loops can exit
                 msg = "Could not fully start listener thread: %s" % (thread_name,)
                 logger.error(msg)
                 raise MessagingRuntimeError(msg)