move loading of testability driver from QApplication to QGuiApplication

This is useful without QWidget too.

Change-Id: Ic7857e52e4a8f57c1205615a1f30323b486814af
Reviewed-by: Richard J. Moore <rich@kde.org>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Alberto Mardegan <mardy@users.sourceforge.net>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>