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.
This test started failing as described in the code comment. May be
related to Nixpkgs updating Fontconfig from version 2.12 to 2.13. See
https://github.com/NixOS/nixpkgs/pull/73795.
This fixes a build error occurring when building a configuration
having fontconfig enabled and `home.packages` only containing one
package installing things to `/lib`.
Also adds a number of test cases to verify the fontconfig cache
generation functionality.
Fixes#703
