Fix: the thread continued running after a test failure or it receives a
SIGINT signal (Ctrl+C).

In order to fix this, this commit set the thread as a daemon. The thread
itself calls _mainloop.run(), which blocks it. Only calling
_mainloop.quit() from the main thread is not enough, so a timeout must be
added when there is no signal in the bus within timeout (changed to 0.5
seconds).