QTest::mouseEvent expects window-local coordinates.

This makes the automatic center click feature of the mouse event methods
also work if the target window isn't positioned at 0,0.

Change-Id: I0d711e484620900ba2ffc4139f7e13346a7482d3
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>

src/testlib/qtestmouse.h

@@ -95,7 +95,7 @@ namespace QTest
         }
 
         if (pos.isNull())
-            pos = window->geometry().center();
+            pos = QPoint(window->width() / 2, window->height() / 2);
 
         QTEST_ASSERT(uint(stateKey) == 0 || stateKey & Qt::KeyboardModifierMask);