QWidgetWindow: Stabilize test on Xcb

Showing, hiding, and showing a window can result in the Xcb QPA plugin
warning about

qt.qpa.xcb: internal error:  void QXcbWindow::setNetWmStateOnUnmappedWindow()
called on mapped window

The point of the test is to verify that we get a paint event on a window
that is shown again after having been hidden, not to verify that async
windowing systems can handle a show/hide/show sequence. So wait for the
window being exposed before we hide it.

Pick-to: 6.2 5.15
Change-Id: If91a9926613645e78e332dacff34bd57e4034b6f
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

tests/auto/widgets/kernel/qwidget_window/tst_qwidget_window.cpp

@@ -465,6 +465,7 @@ void tst_QWidget_window::tst_paintEventOnSecondShow()
 {
     PaintTestWidget w;
     w.show();
+    QVERIFY(QTest::qWaitForWindowExposed(&w));
     w.hide();
 
     w.paintEventCount = 0;