wayfire-stubs: add stubs for wayfire tests

Reducing closure from 500MB to 357MB

tests/modules/services/window-managers/wayfire/configuration.nix

@@ -1,4 +1,6 @@
 { ... }: {
+  imports = [ ./wayfire-stubs.nix ];
+
   wayland.windowManager.wayfire = {
     enable = true;
     package = null;

tests/modules/services/window-managers/wayfire/wayfire-stubs.nix

@@ -0,0 +1,13 @@
+{ config, lib, ... }: {
+  # Avoid unnecessary downloads in CI jobs and/or make out paths constant, i.e.,
+  # not containing hashes, version numbers etc.
+  test.stubs = { wayfire = { }; };
+
+  nixpkgs.overlays = [
+    (_final: _prev: {
+      dbus = config.lib.test.mkStubPackage { name = "dbus"; };
+      systemd =
+        lib.makeOverridable (_attrs: config.lib.test.mkStubPackage { }) { };
+    })
+  ];
+}

tests/modules/services/window-managers/wayfire/wf-shell.nix

@@ -1,4 +1,6 @@
 { pkgs, ... }: {
+  imports = [ ./wayfire-stubs.nix ];
+
   wayland.windowManager.wayfire = {
     enable = true;
     package = null;