test: mark tst_qcombobox as expected failure for qpa

This test fails on qpa because it uses QCursor::setPos, which is
unimplemented.

Task-number: QTBUG-20753
Change-Id: I8b77261e9ddfa8f57cebdbd83c93a367a8cf8bea
Reviewed-on: http://codereview.qt.nokia.com/3128
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Kalle Lehtonen <kalle.ju.lehtonen@nokia.com>

tests/auto/qcombobox/tst_qcombobox.cpp

@@ -2487,6 +2487,10 @@ void tst_QComboBox::keyBoardNavigationWithMouse()
     int selection;
     GET_SELECTION(selection);
 
+#ifdef Q_WS_QPA
+    QEXPECT_FAIL("", "QTBUG-20753 QCursor::setPos doesn't work for qpa", Abort);
+#endif
+
     //since we moved the mouse is in the middle it should even be around 5;
     QVERIFY(selection > 3);