Before using Niv, we were following channels meaning we can not
reproduce CI jobs easily.
In this change, we use Niv to pin these dependencies. We are also
addding a tests/default.nix to be able to run these tests locally.
For instance, to run the test extern.nix on the nixpkgs-19.09 release:
nix-build tests/default.nix -A extern.nixpkgs_19_09
Fixes#178
