This fixes the case where the mixer_module would subscribe and use its
own test_motor publication which was created only to make sure the
topic is advertised and subsequent updates will work properly.
This happened in SITL lockstep because the timestamp would be 0 at the
very beginning, and hence elapsed time would be 0 as well.
This lead to an actuator publication which would then get lockstep out
of sync causing poll errors on the Gazebo side.
ef475fa9d8