nix: add VM tests for flake modules (#2281)

* nix: add VM tests for flake modules

* ci: add NixOS tests

distro/nix/tests/nixos-module.nix

@@ -0,0 +1,47 @@
+{
+  self,
+  pkgs,
+  ...
+}:
+pkgs.testers.runNixOSTest {
+  name = "dms-nixos-module";
+
+  nodes.machine = {
+    imports = [
+      self.nixosModules.dank-material-shell
+    ];
+
+    users.users.danklinux = {
+      isNormalUser = true;
+      extraGroups = [ "wheel" ];
+    };
+
+    programs.dank-material-shell = {
+      enable = true;
+      systemd.enable = true;
+      plugins = {
+        TestPlugin = {
+          src = pkgs.emptyDirectory;
+        };
+      };
+    };
+
+    system.stateVersion = "25.11";
+  };
+
+  testScript = ''
+    import json
+
+    machine.wait_for_unit("multi-user.target")
+
+    machine.succeed("command -v dms")
+    machine.succeed("command -v quickshell")
+    machine.succeed("su -- danklinux -c 'dms --help >/dev/null'")
+    machine.succeed("test -d /etc/xdg/quickshell/dms-plugins")
+    machine.succeed("test -f /run/current-system/sw/lib/systemd/user/dms.service")
+
+    payload = json.loads(machine.succeed("su -- danklinux -c 'dms doctor --json'"))
+    t.assertIn("summary", payload)
+    t.assertIsInstance(payload.get("results"), list)
+  '';
+}