Add `term_taxonomy_id` to available fields in `get_term_by()`. Adds unit test.

Props jchristopher.
Fixes #21651.


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git-svn-id: http://core.svn.wordpress.org/trunk@25296 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Don't call `sanitize_title_with_dashes( $taxonomy )` in `register_taxonomy()` unless `$args['rewrite']['slug']` is empty.

Props SergeyBiryukov.
Fixes #23668.


Built from https://develop.svn.wordpress.org/trunk@25335


git-svn-id: http://core.svn.wordpress.org/trunk@25297 1a063a9b-81f0-0310-95a4-ce76da25c4cd