Skip tst_QGraphicsItem::paint() on Mac OS X 10.7

The test is randomly failing on CI when ran on 10.7.

Task-number: QTBUG-31454
Change-Id: I79fce9a37616c6abaee960e338f8eea8fe6f31cf
Reviewed-by: Jani Heikkinen <jani.heikkinen@digia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>