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

* nix: add VM tests for flake modules

* ci: add NixOS tests

distro/nix/tests/default.nix

@@ -0,0 +1,52 @@
+{
+  self,
+  pkgs,
+  ...
+}:
+rec {
+  all = pkgs.symlinkJoin {
+    name = "dms-nixos-tests";
+    paths = [
+      nixos-module
+      nixos-service-start-module
+      greeter-niri-module
+      niri-home-module
+      home-manager-module
+    ];
+  };
+
+  nixos-module = import ./nixos-module.nix {
+    inherit
+      self
+      pkgs
+      ;
+  };
+
+  nixos-service-start-module = import ./nixos-service-start-module.nix {
+    inherit
+      self
+      pkgs
+      ;
+  };
+
+  greeter-niri-module = import ./greeter-niri-module.nix {
+    inherit
+      self
+      pkgs
+      ;
+  };
+
+  niri-home-module = import ./niri-home-module.nix {
+    inherit
+      self
+      pkgs
+      ;
+  };
+
+  home-manager-module = import ./home-manager-module.nix {
+    inherit
+      self
+      pkgs
+      ;
+  };
+}