tests: Add a test for the nixos module (#907)

flake-modules/tests.nix

@@ -6,11 +6,13 @@
   perSystem = {
     pkgs,
     config,
+    system,
     makeNixvimWithModuleUnfree,
     makeNixvimWithModule,
     ...
   }: {
-    checks = {
+    checks =
+      {
         tests = import ../tests {
           inherit pkgs;
           inherit (pkgs) lib;
@@ -32,13 +34,25 @@
           inherit (pkgs) lib;
         };
 
-      home-manager =
+        home-manager-module =
           (import ../tests/modules/hm.nix {
             inherit pkgs;
             inherit (inputs) home-manager;
             nixvim = self;
           })
           .activationPackage;
+      }
+      // pkgs.lib.optionalAttrs (!pkgs.stdenv.isDarwin) {
+        nixos-module =
+          (import ../tests/modules/nixos.nix {
+            inherit system;
+            inherit (inputs) nixpkgs;
+            nixvim = self;
+          })
+          .config
+          .system
+          .build
+          .toplevel;
       };
   };
 }

tests/modules/nixos.nix

@@ -0,0 +1,21 @@
+{
+  nixvim,
+  nixpkgs,
+  system,
+}:
+nixpkgs.lib.nixosSystem {
+  inherit system;
+
+  modules = [
+    {
+      system.stateVersion = "23.11";
+      boot.loader.systemd-boot.enable = true;
+      fileSystems."/" = {device = "/non/existent/device";};
+
+      programs.nixvim = {
+        enable = true;
+      };
+    }
+    nixvim.nixosModules.nixvim
+  ];
+}