Prospective stabilization fix for tst_qwindow::positioning(default)

Our theory for the failure of framePosition() not having the expected
value after setFramePosition towards the end of the test is that we try
to call setFramePosition() while the getting-back-from-fullscreen-to-normal
window animation is still running, at which point the compositor may
just choose to ignore our move request.

Similarly to when going fullscreen, also wait when coming back from it.

Change-Id: Icfc92f277d96dccdfad772c4aac252b2a20c6196
Reviewed-by: Jørgen Lind <jorgen.lind@theqtcompany.com>