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.
Fixes tests that are affected by <https://github.com/NixOS/nixpkgs/pull/333744>.
extraOptions
Allows a user to, for example, add the `-g` option. Also add a basic test case.