NixOS-backed graphical integration testing

[diamondburned]

Using NixOS' testing infrastructure, we can add a test that ensures the app runs properly via a graphical VM. This also gives us a free interactive VM that we can use to generate screenshots.

Useful links:

• https://github.com/NixOS/nixpkgs/blob/master/nixos/tests/chromium.nix
• https://discourse.nixos.org/t/nixos-integration-tests-with-graphical-applications-best-practice/11617/6