Tests: Split the tests from user/author.php into individual test classes.
user/author.php
This aims to make get_the_author_*() tests more discoverable and easier to expand.
get_the_author_*()
Follow-up to [691/tests], [692/tests], [695/tests], [28362], [31098], [34687], [53147], [54717], [54726], [55591].
See #57841.