tst_QWindow::positioning fails on Wayland

The test fails on QtWayland, mostly due to the fact that Wayland doesn't tell us where on the screen a window is placed.

The test should probably just be skipped, but not based on the platform name. Some Wayland shell extension may be added that implement support for this. A proper solution would probably involve creating a platform capability for the ability to know or set the window position, which we should set correctly in Qt Wayland.