[autotests] Add test case for fullscreen window with groups
When an X11 window is raised to fullscreen it gets set to active layer.
When another window gets activated then it goes back to normal layer.
But when a window of the same group gets activated the fullscreen window
stays in the active layer. Due to that it is not possible to raise other
windows above the fullscreen window.
This just adds a test case exposing the problematic area.