Fix regression causing all taxonomies to be returned for all types

While the taxonomies routes are now type-independent, the post type
routes (e.g. /posts/types/post) return embedded taxonomy objects for
each taxonomy associated with that type. Removing the "types" argument
from the get_taxonomies method caused a regression where ALL
taxonomies would be returned for any post type endpoint.
Add test for add_taxonomy_data regression

This adds a test to verify that get_taxonomies only returns taxonomies
for the provided post type if a type is passed into the function. This
ensures that the taxonomies embedded into post type endpoint responses
will be accurate.

It also DRYs up the test code a bit to avoid repeating the methods for
filtering a taxonomy collection down to only "public" members.
WIP