In unit tests, use `wpmu_delete_user()` to delete users when `is_multisite()`.

`wp_delete_user()` doesn't remove the user from the database, which causes all
sorts of problems in certain cases.

Props jeremyfelt.
See [30277], #30017.
Built from https://develop.svn.wordpress.org/trunk@30278


git-svn-id: http://core.svn.wordpress.org/trunk@30278 1a063a9b-81f0-0310-95a4-ce76da25c4cd