Deprecate admin_created_user_subject()

When `admin_created_user_subjec()` was merged from MU, the accompanying filter was left behind. As it has never been used by WordPress core, and is not an otherwise useful function, it can be deprecated.

Fixes #29915

Built from https://develop.svn.wordpress.org/trunk@30005


git-svn-id: http://core.svn.wordpress.org/trunk@30005 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Begin cleanup of ms-sites group unit tests

* Split `test_create_and_delete_blog()` into two tests.
* Clean up number of sites created during tests.
* Remove test_getters(), move get_blog_address_by_name() test to `test_created_site_details()`

See #30080

Built from https://develop.svn.wordpress.org/trunk@30006


git-svn-id: http://core.svn.wordpress.org/trunk@30006 1a063a9b-81f0-0310-95a4-ce76da25c4cd