Add testing

[hedning]

We want to add tests (perhaps CI if it's possible too).

One way to do testing is setting XDG_CONFIG_HOME, populating it with a user.js which then would run the tests.

A simple test for something like fullscreen reactivity (which I broke twice recently):

• create a window, wait for it to appear
• set fullscreen
• check that actor.visible == true and clone.visible == false
• signal completion to the test runner, with eg. emit('done', [ok, result])

A more complex example would be restarts on x11:

• create some windows
• save some state
• restart
• check that we've restarted
• signal completion

[Thesola10]

This could definitely build upon #982