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/feh/feh-bindings.nix

@@ -17,8 +17,7 @@
       prev_img = [ "h" "Left" ];
     };
 
-    nixpkgs.overlays =
-      [ (self: super: { feh = pkgs.writeScriptBin "dummy-feh" ""; }) ];
+    test.stubs.feh = { };
 
     nmt.script = ''
       assertFileContent \

tests/modules/programs/feh/feh-empty-settings.nix

@@ -4,8 +4,7 @@
   config = {
     programs.feh.enable = true;
 
-    nixpkgs.overlays =
-      [ (self: super: { feh = pkgs.writeScriptBin "dummy-feh" ""; }) ];
+    test.stubs.feh = { };
 
     nmt.script = ''
       assertPathNotExists home-files/.config/feh/buttons