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

* nix: add VM tests for flake modules

* ci: add NixOS tests

distro/nix/tests/greeter-niri-module.nix

@@ -0,0 +1,60 @@
+{
+  self,
+  pkgs,
+  ...
+}:
+pkgs.testers.runNixOSTest {
+  name = "dms-greeter-niri-module";
+
+  nodes.machine = {
+    imports = [
+      self.nixosModules.greeter
+    ];
+
+    users.groups.greeter = { };
+    users.users.greeter = {
+      isSystemUser = true;
+      group = "greeter";
+    };
+
+    services.greetd.settings.default_session.user = "greeter";
+
+    programs.niri.enable = true;
+
+    programs.dank-material-shell.greeter = {
+      enable = true;
+      compositor.name = "niri";
+    };
+
+    system.stateVersion = "25.11";
+  };
+
+  testScript = ''
+    import re
+
+    machine.wait_for_unit("multi-user.target")
+    machine.wait_for_unit("greetd.service")
+
+    machine.succeed("systemctl is-enabled greetd.service")
+    machine.succeed("systemctl is-active greetd.service")
+
+    greetd_unit = machine.succeed("cat /etc/systemd/system/greetd.service")
+    config_match = re.search(r'--config (/nix/store[^ ]+-greetd.toml)', greetd_unit)
+    if config_match is None:
+        raise AssertionError(greetd_unit)
+
+    greetd_config_path = config_match.group(1)
+    greetd_config = machine.succeed(f"cat {greetd_config_path}")
+    t.assertIn("dms-greeter", greetd_config)
+
+    script_match = re.search(r'command\s*=\s*"([^"]+/bin/dms-greeter)"', greetd_config)
+    if script_match is None:
+        raise AssertionError(greetd_config)
+
+    script_path = script_match.group(1)
+    script = machine.succeed(f"cat {script_path}")
+    t.assertIn("--command", script)
+    t.assertIn("niri", script)
+    t.assertIn("/share/quickshell/dms", script)
+  '';
+}