Implement a 'desktop testing' interface
UI testing tools have traditionally relied on the global positions and the event injection capabilities of the client-side a11y interface. With Wayland, that is not possible, since clients simply don't have global positions, and we don't want to allow event injection in the sandboxed case.
Mutter should provide a suitable D-Bus api that priviledged, non-sandboxed test tools can use as a replacement.