Factored out nixos-test into a separate file

[toastal]

Not entirely sure on the value here after exploration…

Once I started down the rabbit hole of trying to pass around configuration that wasn’t for static files (meaning a more complicated configuration), I realized that one would need a lot of unnecessary configuration to insert your own test code & what have you in the middle of the test like adding hooks for the Python script for more ports & other code to handle before running the test… to which at that point, I think folks would be better off just giving credit & to writing their own NixOS tests based on this instead of using it (at least not without a major refactor/rethinking of the project).

What is here would allow someone to more easily import & use the file outside of Flake modules tho.

Nevertheless, here was the start of it which seems to build. No hard feelings if you just close (or cherrypick).