Skip unstable tst_QMdiArea::subWindowActivated2 test on Mac OS X.

This test function recently became unstable in parallel with the change
to re-enable the test.  Skip the unstable function until it can be
fixed.

Change-Id: I0fb962aa3294a0c9b799acee52ab1fb1cd1c6924
Reviewed-by: Sergio Ahumada <sergio.ahumada@nokia.com>

tests/auto/widgets/widgets/qmdiarea/tst_qmdiarea.cpp

@@ -531,7 +531,7 @@ void tst_QMdiArea::subWindowActivated2()
     QSKIP("Not fixed yet. See Task 197453");
 #endif
 #ifdef Q_OS_MAC
-    QEXPECT_FAIL("", "QTBUG-25298", Abort);
+    QSKIP("QTBUG-25298: This test is unstable on Mac.");
 #endif
     QTRY_COMPARE(spy.count(), 1);
     QVERIFY(!mdiArea.activeSubWindow());