nix-build failed because the tests assume to run in a CWD equal to the
project root, which is not the case in the derivation build.
This commit fixes it by not using hacky `..` references to paths,
and instead uses NIX_PATH for all implicit Nix testing path
dependencies.
Also the root of the `lib` path gets passed in from the `default.nix`
file, so all the relative path handling is done by Nix during evaluation
already, and in the Nix store when possible.
Allows detecting whether attributes are overridden in all-packages.nix.
In a future commit we'll use this to detect empty arguments being set in
all-packages.nix and complain about that.
