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

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git-svn-id: http://core.svn.wordpress.org/trunk@30399 1a063a9b-81f0-0310-95a4-ce76da25c4cd