[autotests] Try to make SceneQPainterTest::testX11Window more robust

Still failing on CI, let's try  to verify that the buffer is ready.

autotests/integration/scene_qpainter_test.cpp

@@ -35,6 +35,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #include <KWayland/Client/shell.h>
 #include <KWayland/Client/surface.h>
 #include <KWayland/Client/pointer.h>
+#include <KWayland/Server/surface_interface.h>
 
 #include <QPainter>
 
@@ -339,6 +340,7 @@ void SceneQPainterTest::testX11Window()
         QVERIFY(surfaceChangedSpy.wait());
     }
     QVERIFY(client->surface());
+    QTRY_VERIFY(client->surface()->buffer());
 
     // enough time for rendering the window
     QTest::qWait(100);