Introducing NixOS end-to-end test framework

[Thesola10]

The time has finally come! This system should allow interactive tests by simulating the mouse and keyboard and comparing against screenshots (and maybe also ensure GNOME hasn't crashed).

Also fixes #91 and #1098

To-do list:

• Wire Behave into NixOS test system
• Play back input devices using libinput
• Run GJS code within GNOME Shell
• Wait until PaperWM has finished loading
• Take screenshots using OpenCV
• Produce test report artifact with screenshots attached
• Extract useful information such as window positions from OpenCV
• Write a complete, meaningful test
• Stretch goal: obtain coverage data

[Thesola10]

I'd love to introduce code coverage as well as GJS seems to support it but with GNOME being welded together I can't even begin to figure out how to use it right now.

[Thesola10]

I think I'll merge this once the test functions all work and start implementing the tests themselves in a separate PR.