Feature/documentation request: testing story

[shanesveller]

Preface: I am a fairly casual nixpkgs contributor thus far and have not submitted any NixOS modules or tests upstream, and in general I am woefully unfamiliar with my opportunities and available techniques to test Nix code in any context. I know there's some prior art for me to review in the form of the NixOS testing support, and that's about all I know today.

As mentioned elsewhere, I'm working on my first non-trivial flake intended for public consumption, and it's defined in terms of a flake-parts module. I'd like to keep a few common permutations around in-repo and evaluate them with/against my current flakeModule content to prove that they still work as intended and check some simple results. Is this something that I can do today? Would such an approach be best based around lib.evalFlakeModule?

Early motivating examples of "unit tests" I'd like to incorporate are:

• If I set a particular option defined by my flakeModule, does a particular leaf node of config hold an expected value?
• If I set a particular option defined by my flakeModule, does the consuming flake now have an output at a particular path?

Later on it would be cool to also check things like:

• Is such-and-such binary in PATH in the generated devShell?
• Does a particular shell command produce expected output when run in the context of the devShell?

As with #77, if some early guidance is available I'd be happy to try to implement this myself. I am not always able to deeply comprehend some of the more sophisticated code in the Nix ecosystem without assistance - my functional-programming background begins and ends with Elixir.