How to test custom module produces specific result?

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I have custom modules that I enable for a NixOS system. E.g.

# modules/nixos/tools/direnv/default.nix
{ options, config, lib, pkgs, namespace, ... }:
with lib;
with lib.${namespace}; let
	cfg = config.${namespace}.tools.direnv
in {
	options.${namespace}.tools.direnv = with types; {
		enable = mkBoolOpt false "Whether or not to enable direnv.";
	};

	config =
		mkIf cfg.enable {
			mypkgs.home.extraOptions = {
				programs.direnv = {
					enable = true;
					nix-direnv = enabled;
				};
			};
		};
}

I enable this in my systems configuration like so, options.mypkgs.tools.direnv.enable = true;

I then want to update the code of the module to have more arguments, or optimise in some way, etc.

# updated modules/nixos/tools/direnv/default.nix
{ options, config, lib, pkgs, namespace, ... }:
with lib;
with lib.${namespace}; let
	cfg = config.${namespace}.tools.direnv;
in {
	options.${namespace}.tools.direnv = with types; {
		enable = mkBoolOpt false "Whether or not to enable direnv.";
		# Added new option
		foobar.enable = mkBoolOpt false "Whether or not to enable direnv foobar.";
	};

	config =
		mkIf cfg.enable {
			mypkgs.home.extraOptions = {
				programs.direnv = {
					enable = true;
					# Added config for new option
					foobar mkIf cfg.foobar.enable = true;
				};
			};
		};
}

How can I check that given the same inputs to the options I get the same result? In this example, I’m checking that the old systems don’t need to all be updated with foobar.enable = false;

I’ve looked at a few things, including namaka which suggested it could do snapshot testing like this, but I couldn’t get it to work for the nixosConfiguration. Nix eval can do it at some levels of the flake, but not the whole config, and a wrapper around that didn’t feel scalable.