Event of type restart could not be handled properly, because of
its special behavior. For handling this event in most natural way
we recursively call `do_events_register` when restart event is
recieved, based on custom error created for this event.
Testing: Second call to get_user_events due to recursive calling
of do_event_register, is expected to not contain the restart event.
So new test added in test_event_system.py are based on above behavior
of get_user_events.
Fixes: #15541.