All Nixpkgs versions are tested with same generated files

[sellout]

This causes an issue when different versions of packages in different Nixpkgs releases have different file formats.

E.g., 23.11 has Vale 2.29 and 24.05 has Vale 3.0, which changes the location of vocabularies. We need to generate both for all the tests to pass. But in general projects would only be using one version of Vale, and would only want those files generated.