message: fix heartbeat test
diff: @@ -120,8 +120,9 @@ def test_monitor_heartbeats_remove_participant(_mock_sleep, _mock_event):
  def test_participant_heartbeat(mock_heartbeat_request, _mock_sleep, _mock_event):
  channel = mock.MagicMock()
  terminate_event = threading.Event()
  + selected_event = threading.Event()
  - heartbeat(channel, terminate_event)
  + heartbeat(channel, terminate_event, selected_event)
  # check that the heartbeat is sent exactly twice
  mock_heartbeat_request.assert_has_calls([mock.call(), mock.call()])
  