This isn't nix-shell. nix-shell is a tool for spawning ephemeral software environments composed of nix packages, something a little like python's virtualenvs.

Digging a little bit, nixos' tests appear to be written in perl and are executed in a context that has access to a bunch of useful primitives for asserting these sorts of things. https://nixos.org/nixos/manual/#sec-writing-nixos-tests