autotest: ensure no stray message listeners after test

Tools/autotest/common.py

@@ -5784,6 +5784,8 @@ def run_one_test_attempt(self, test, interact=False, attempt=1, do_fail_list=Tru
 
         tee = TeeBoth(test_output_filename, 'w', self.mavproxy_logfile)
 
+        start_num_message_hooks = len(self.mav.message_hooks)
+
         prettyname = "%s (%s)" % (name, desc)
         self.start_test(prettyname)
         self.set_current_test_name(name)
@@ -5856,6 +5858,12 @@ def run_one_test_attempt(self, test, interact=False, attempt=1, do_fail_list=Tru
                           (old_contexts_length, len(self.contexts)))
             passed = False
 
+        # make sure we don't leave around stray listeners:
+        if len(self.mav.message_hooks) != start_num_message_hooks:
+            self.progress("Stray message listeners: %s" %
+                          str(self.mav.message_hooks))
+            passed = False
+
         if passed:
 #            self.remove_bin_logs() # can't do this as one of the binlogs is probably open for writing by the SITL process.  If we force a rotate before running tests then we can do this.  # noqa
             pass