When adding queries to `tax_query`: if the query's `field` is `term_taxonomy_id`, don't require `taxonomy` to be specified. In `WP_Tax_Query::transform_query()`, `$query['taxonomy']` is never checked for the 'term_taxonomy_id' `case` because 'term_taxonomy_id' is the primary key being looked up.

Adds unit tests.

Props helen.
Fixes #25284.



git-svn-id: https://develop.svn.wordpress.org/trunk@28562 602fd350-edb4-49c9-b593-d223f7449a82