[autotest] Try to make TestWaylandOutputDevice a little bit more robust

From time to time the test is failing on build.kde.org with an ASAN
heap-use-after-free error. From my investigation this seems to be caused
by the OutputDevice being constructed on the stack and being destroyed
while handling Wayland events, but before all are handled.

The test mostly operates on the changed signal. There is also a done
signal emitted later on. Wayland sends the done after a set of changes
is transmitted. Thus the test is adjusted to wait for done instead of
changed. So we can ensure that all events are handled before the object
gets destroyed.

I have never been able to reproduce the problem locally, so I cannot
guarantee that the issue is solved for good. If it still happens more
investigation will be needed.