Fake mouse_move when using real windows in tests

[itziakos]

Currently the enable test assistant supports faking a mouse_move operation using a MockWindow or a real window with mocked get_pointer_position. It might be possible to properly implement the mouse_move without the need for mocking get_pointer_position when real windows are used.

[corranwebster]

This is almost certainly possible, but will require toolkit-specific infrastructure.