Add tests starting GUIs, likely falling back to xvfb for it

[andypugh]

See discussion in this separate issue: #3391 (comment)

[smoe]

When starting LinuxCNC, this typically comes with a GUI. But anything that needs to open a Window typically terminates when there is no X display server running. However, when executed after "xvfb-run", there is no such complaint. This way it should be possible to get our GUIs up, perform various actions, and possibly also to terminate again in a controlled manner.

I think it was @petterreinholdtsen who prepared such tests (without Xvfb) in debian/tests, which I would very much like to see developed further.