Stabilize tst_qabstractitemview.

Center windows on screen to avoid conflicts with Unity taskbars,
observe minimum geometry for Windows, move cursor out of the
way for Mac. Introduce QScopedPointer to ensure cleanup in
case of failures.

Change-Id: Ic169e015e795597457b8e85a28ff7fcb7b9b9430
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>