Skip flaky test on MacOS_10_12

This is the most flaky-pass test currently.
It fails the first time it is run on MacOS_10_12, but
succeeds all the following times.
This happens extremely often, so disable it until the issue
is resolved.

Task-number: QTBUG-76566
Change-Id: I94359eceb91c3b958930424e6c8b5957fb3f1252
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

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

@@ -5049,6 +5049,12 @@ public:
 
 void tst_QGraphicsItem::paint()
 {
+#if defined(Q_OS_MACOS)
+    if (QSysInfo::productVersion() == QLatin1String("10.12")) {
+        QSKIP("Test is very flaky on MacOS_10_12, see QTBUG-76566");
+    }
+#endif
+
     QGraphicsScene scene;
 
     PaintTester paintTester;