Fix tst_qquickview for full screen platforms

Some test cases rely on the fact that show() is not fullscreen, which may not
be true for some platforms. Explicitly make use of showNormal() to avoid
full-screen show on these platforms.

Change-Id: I0787fb62b8e37a73974f87857de200485d7ba6da
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>