Upload:

- Fix PHP warnings in `wp_unique_filename()` when the destination directory is unreadable.
- Run the final name collision test only for files that are saved to the uploads directory.
- Update the unit tests to match.

Props eden159, audrasjb, azaozz.
Fixes #48960 for trunk.
Built from https://develop.svn.wordpress.org/trunk@46965


git-svn-id: https://core.svn.wordpress.org/trunk@46765 1a063a9b-81f0-0310-95a4-ce76da25c4cd