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

* nix: add VM tests for flake modules

* ci: add NixOS tests

distro/nix/tests/nixos-service-start-module.nix

@@ -0,0 +1,48 @@
+{
+  self,
+  pkgs,
+  ...
+}:
+let
+  fakeDms = pkgs.writeShellScriptBin "dms" ''
+    printf '%s\n' "$@" > /tmp/dms-service-args
+    exec ${pkgs.coreutils}/bin/sleep 300
+  '';
+in
+pkgs.testers.runNixOSTest {
+  name = "dms-nixos-service-start-module";
+
+  nodes.machine = {
+    imports = [
+      self.nixosModules.dank-material-shell
+    ];
+
+    users.users.danklinux = {
+      isNormalUser = true;
+      linger = true;
+      extraGroups = [ "wheel" ];
+    };
+
+    programs.dank-material-shell = {
+      enable = true;
+      package = fakeDms;
+      systemd = {
+        enable = true;
+        target = "default.target";
+      };
+    };
+
+    system.stateVersion = "25.11";
+  };
+
+  testScript = ''
+    machine.wait_for_unit("multi-user.target")
+    machine.wait_for_unit("user@1000.service")
+
+    machine.succeed("systemctl --machine=danklinux@ --user start dms.service")
+    machine.wait_until_succeeds("systemctl --machine=danklinux@ --user is-active dms.service")
+    machine.wait_until_succeeds("test -f /tmp/dms-service-args")
+    machine.succeed("grep -Fx run /tmp/dms-service-args")
+    machine.succeed("grep -Fx -- --session /tmp/dms-service-args")
+  '';
+}