tests: rework derivation stubbing

Instead of having to manually stub packages that should not be
downloaded we instead automatically stub all packages (except a small
list of whitelisted ones). Tests can re-introduce the real package by
using the `realPkgs` module argument.

tests/modules/programs/kitty/theme-to-themeFile.nix

@@ -1,24 +1,24 @@
-{ config, lib, pkgs, options, ... }: {
-  config = {
-    programs.kitty = {
-      enable = true;
-      theme = "Space Gray Eighties";
-    };
+{ lib, options, realPkgs, ... }:
 
-    test.stubs.kitty = { };
-
-    test.asserts.warnings.enable = true;
-    test.asserts.warnings.expected = [
-      ("The option `programs.kitty.theme' defined in ${
-          lib.showFiles options.programs.kitty.theme.files
-        } has been changed to `programs.kitty.themeFile' that has a different"
-        + " type. Please read `programs.kitty.themeFile' documentation and"
-        + " update your configuration accordingly.")
-    ];
-
-    nmt.script = ''
-      assertFileExists home-files/.config/kitty/kitty.conf
-      assertFileRegex home-files/.config/kitty/kitty.conf "^include .*themes/SpaceGray_Eighties\.conf$"
-    '';
+{
+  programs.kitty = {
+    enable = true;
+    theme = "Space Gray Eighties";
   };
+
+  test.asserts.warnings.enable = true;
+  test.asserts.warnings.expected = [
+    ("The option `programs.kitty.theme' defined in ${
+        lib.showFiles options.programs.kitty.theme.files
+      } has been changed to `programs.kitty.themeFile' that has a different"
+      + " type. Please read `programs.kitty.themeFile' documentation and"
+      + " update your configuration accordingly.")
+  ];
+
+  nixpkgs.overlays = [ (self: super: { inherit (realPkgs) kitty-themes; }) ];
+
+  nmt.script = ''
+    assertFileExists home-files/.config/kitty/kitty.conf
+    assertFileRegex home-files/.config/kitty/kitty.conf "^include .*themes/SpaceGray_Eighties\.conf$"
+  '';
 }