Upload: fix `wp_unique_filename()` to prevent name collisions with existing or...

Upload: fix `wp_unique_filename()` to prevent name collisions with existing or future image sub-size file names, and add unit tests.

Props Viper007Bond, pbiron, azaozz.
Fixes #42437.
Built from https://develop.svn.wordpress.org/trunk@46822


git-svn-id: https://core.svn.wordpress.org/trunk@46622 1a063a9b-81f0-0310-95a4-ce76da25c4cd