Get by and return `term_taxonomy_id`

It's our canonical reference to the term
Group these tests for easy running
Revert "Group these tests for easy running"

This reverts commit 8004390d838be12317ffce437f8c04d8f42c62a0.
Merge pull request #648 from WP-API/639-term-id

Get by and return `term_taxonomy_id`