tests: update default arguments

The setup's tentative. Unfortunately, still requires the right
experimental features enabled.

tests/default.nix

@@ -1,8 +1,16 @@
-{ pkgs ? import <nixpkgs> { } }:
+let
+  flakeUtils = import ../lib/flake.nix;
+  flake = flakeUtils.importFlakeMetadata ../flake.lock;
+in
+{ pkgs ? import (flakeUtils.fetchTree flake "nixos-unstable") { } }:
 
+let
+  utils = import ./utils.nix { inherit pkgs; };
+in
 {
-  lib = import ./lib { inherit pkgs; };
+  lib = import ./lib { inherit pkgs utils; };
   modules = {
-    home-manager = import ./modules/home-manager { inherit pkgs; };
+    home-manager = import ./modules/home-manager { inherit pkgs utils; };
+    nixos = import ./modules/nixos { inherit pkgs utils; };
   };
 }

tests/lib/default.nix

@@ -1,6 +1,6 @@
 # For the environment-specific subset, we'll be simulating the configurations
 # as a simple attribute set since that's what they are anyways.
-{ pkgs ? import <nixpkgs> { } }:
+{ pkgs ? import <nixpkgs> { }, utils ? import ../utils.nix { inherit pkgs; } }:
 
 let
   inherit (pkgs) lib;

tests/utils.nix

@@ -0,0 +1,20 @@
+# A set of functions to facilitate testing in the project.
+{ pkgs }:
+
+let
+  nixpkgsPath = pkgs.path;
+  nixosLib = import "${nixpkgsPath}/nixos/lib" { };
+in
+rec {
+  # We're not using this to test the hosts configuration (that would be
+  # atrocious). We're only using this for NixOS modules.
+  nixosTest = test:
+    nixosLib.runTest {
+      imports = [ test ];
+      hostPkgs = pkgs;
+      specialArgs = {
+        foodogsquaredUtils = import ../lib/utils/nixos.nix { inherit pkgs; };
+        foodogsquaredModulesPath = builtins.toString ../modules/nixos;
+      };
+    };
+}