Plug memleaks in tst_QWidget

We need to delete the style returned from QStyleFactory::create()
ourselves, so put them into a QScopedPointer.

The alternative would have been to create this once, as a member
of tst_QWidget, but this is the minimal approach that ensures
behavior just as the old code, but without the leak.

Change-Id: I527f1031c57be6f05942f4acc057e7dae1af2571
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>