Comment out some CSS files in `$_old_files` that were added back as a result of [36341].

Add a unit test to make sure the `$_old_files` array does not contain any current project files.

Props joemcgill, swissspidy.
Fixes #36083.
Built from https://develop.svn.wordpress.org/trunk@36843


git-svn-id: https://core.svn.wordpress.org/trunk@36810 1a063a9b-81f0-0310-95a4-ce76da25c4cd