When the Single Frame Mode is set to NEXT, stbt must be already waiting for
the next message in the bus. Otherwise, it gets lost, and stbt gets blocked
when it reaches the line:

_mainloop.run()

That is a very particular case that happens around 1 out of 90
test runs. This has been fixed by running the _mainloop in a different
thread, and therefore, making sure that when the Single Frame Mode is
changed, the the message is received.