NixOS first impressions: writing system-level tests

My motivation for NixOS I've been using Nix for some time now. I converted my dotfiles to mostly Nix, and I'm very happy with how useful it became. The next logical step is NixOS, that applies the same principles to the whole OS. I've been thinking about how I'll use NixOS for a while and now is the time to actually give it a go. Usually when people try out an operating system, they go the usual…