Stabilize QGraphicsWidget test.

The test showed frequent failures with XCB in
initialShow2() (line 3200) after
01bc3408

 (QTBUG-30923) which
can be fixed by ensuring the widget from the HFW-test is deleted.

Use QScopedPointer to ensure widgets are always deleted in case
of test failures, too.

Task-number: QTBUG-20778
Task-number: QTBUG-30923
Change-Id: I2af7737b604820463f760d6b6787dd5a5a93d602
Reviewed-by: Jan Arve Sæther <jan-arve.saether@digia.com>