Prevent wpmu_delete_blog from removing the wrong uploads directory

`wp_upload_dir()` includes some logic to fall back to the default site's upload directory if a specific directory for the requested site cannot be found. Because of this, if `wpmu_delete_blog()` is fired twice in a row for the same site, the main site's upload directory could be deleted as well.

This adds some checks in `wpmu_delete_blog()` so that we are confident in the site and it's upload directory's existence before dropping the site. Tests are added for when `ms_files_rewriting` is enabled or disabled.

Fixes #30121


git-svn-id: https://develop.svn.wordpress.org/trunk@30404 602fd350-edb4-49c9-b593-d223f7449a82