Switch derivation test from -core to hspec(-golden) #261

It is the only remaining test using tasty and tasty-golden.

There's also a problem with it - it works when run via cabal test core but not via cabal run core as the directory prefix is in the way and it has trouble finding fixture(s). Similar problem was resolved in -nar https://github.com/haskell-nix/hnix-store/blob/master/hnix-store-nar/tests/NarFormat.hs#L171-L187 where it tries both with and without a subdir.