Harmonize cookbook responses between Ruby / Erlang

We had a few helpers ('minimal_cookbook', 'minimal_response',
'retrieved_cookbook') that returned "minimal" versions of a cookbook…
those have now been merged together into a single 'retrieved_cookbook'
method that respects the differences between Ruby and Erlang
endpoints.  This new method is written to make it more clear what the
exact differences between the implementations are.

Tests now all use this helper.  Since the depsolver endpoint is unique
(in that the Erlang implementation removes some additional metadata
fields), I've just removed those fields in the test itself, since it's
only used once.

Remove make_minimal_cookbook helper, too, since it wasn't used.