Use correct file name in a hook comment.

props wedi.
fixes #30200.
Built from https://develop.svn.wordpress.org/trunk@30117


git-svn-id: http://core.svn.wordpress.org/trunk@30117 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Move `term_exists()` tests to their own file.
Built from https://develop.svn.wordpress.org/trunk@30118


git-svn-id: http://core.svn.wordpress.org/trunk@30118 1a063a9b-81f0-0310-95a4-ce76da25c4cd