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.


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