nixpkgs/nixos/doc/manual/development/nixos-tests.chapter.md

NixOS Tests

When you add some feature to NixOS, you should write a test for it. NixOS tests are kept in the directory nixos/tests, and are executed (using Nix) by a testing framework that automatically starts one or more virtual machines containing the NixOS system(s) required for the test.

writing-nixos-tests.section.md
running-nixos-tests.section.md
running-nixos-tests-interactively.section.md
linking-nixos-tests-to-packages.section.md
testing-hardware-features.section.md