qquicktest: Wait for the view to finish loading

qquicktest: Wait for the view to finish loading

Tests can be skipped if the view is still loading by the time we get
to the end of the loop. There is a case where the window is active
but the QML hasn't been loaded.

Change-Id: I4e8346ee547653810458d042925d673748c1fec8
Reviewed-by: Caroline Chao <caroline.chao@theqtcompany.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: J-P Nurmi <jpnurmi@theqtcompany.com>