tests: add option test.stubs

This option provides a more convenient way to overlay dummy packages.
It also adds a function `config.lib.test.mkStubPackage` that can,
e.g., be used for `package` options.

tests/modules/programs/bat/bat.nix

@@ -18,8 +18,7 @@ with lib;
       '';
     };
 
-    nixpkgs.overlays =
-      [ (self: super: { bat = pkgs.writeScriptBin "dummy" ""; }) ];
+    test.stubs.bat = { };
 
     nmt.script = ''
       assertFileExists home-files/.config/bat/config