Accept 'orderby=include' in `WP_User_Query`.

This lets the results of a user query be sorted manually by the value of the
'include' param.

Props jipmoors.
Fixes #30064.

git-svn-id: https://develop.svn.wordpress.org/trunk@30016 602fd350-edb4-49c9-b593-d223f7449a82
Streamline `WP_User_Query` unit tests.

* Don't create user during setUp(), as it's not used in every test.
* Create fewer users in `get_all` and `orderby` tests.

See #30017.

git-svn-id: https://develop.svn.wordpress.org/trunk@30017 602fd350-edb4-49c9-b593-d223f7449a82