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>

tests/auto/widgets/graphicsview/qgraphicsitem/tst_qgraphicsitem.cpp

@@ -5089,6 +5089,10 @@ public:
 
 void tst_QGraphicsItem::paint()
 {
+#ifdef Q_OS_MACX
+    if (QSysInfo::MacintoshVersion == QSysInfo::MV_10_7)
+        QSKIP("QTBUG-31454 - Unstable auto-test");
+#endif
     QGraphicsScene scene;
 
     PaintTester paintTester;