Make tst_qwidget more stable

This change fixes a failure of tst_qwidget under XFCE (Ubuntu 13.04).

Change-Id: Ic63e85a97ac6e44ce8c281a80ef4e83feecd9185
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>

tests/auto/widgets/kernel/qwidget/tst_qwidget.cpp

@@ -5626,6 +5626,7 @@ void tst_QWidget::showHideShowX11()
     qApp->installNativeEventFilter(&w);
 
     w.show();
+    QVERIFY(QTest::qWaitForWindowExposed(&w));
     w.hide();
 
     QEventLoop eventLoop;