Add tests to Nix packaging

It's possible to run the test suite using stack test or stack build with the right arguments. This works alright and is relatively fast if you have a .stack-work with all the dependencies cached. If you don't then it's a long wait to build everything.

iohk haskell.nix supports building/running tests too. It does about the same thing as stack test but uses the Nix build cache. The Nix cache is a bit better than the .stack-work cache because it is more fine-grained and less apt to get accidentally wiped.

It would be great to have both these options to increase the chances of having a usable cache around.