spectrwm-stubs: add stubs for spectrwm tests

Reducing closure from 500MB to 357MB

tests/modules/services/window-managers/spectrwm/spectrwm-simple-config.nix

@@ -1,6 +1,8 @@
 { ... }:
 
 {
+  imports = [ ./spectrwm-stubs.nix ];
+
   xsession.windowManager.spectrwm = {
     enable = true;
     settings = {

tests/modules/services/window-managers/spectrwm/spectrwm-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 = { spectrwm = { }; };
+
+  nixpkgs.overlays = [
+    (_final: _prev: {
+      dbus = config.lib.test.mkStubPackage { name = "dbus"; };
+      systemd =
+        lib.makeOverridable (_attrs: config.lib.test.mkStubPackage { }) { };
+    })
+  ];
+}