Split and organize multisite unit tests

* Move `ms.php` to `multisite.php`
* Create `multisite.php` under directories `option/` and `user/` to better match existing structure.
* Create a `multisite/` directory containing `bootstrap.php`, `site.php`, and `network.php` for very multisite specific testing.
* Add unit test groups ms-site, ms-user, ms-option, ms-network, and ms-bootstrap.

Fixes #29896


git-svn-id: https://develop.svn.wordpress.org/trunk@29916 602fd350-edb4-49c9-b593-d223f7449a82