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.

git-svn-id: https://develop.svn.wordpress.org/trunk@30278 602fd350-edb4-49c9-b593-d223f7449a82