External Libraries: Test that MediaElement SWF files remain deleted.
The files were removed from Core in r42462 because they're no longer necessary, and have a history of security issues. They remain upstream, though, so this test makes it explicitly clear that they should not be added back in the future without careful consideration and discussion with the Security team.
Tests_Admin_IncludesUpdateCore::test_new_files_are_not_in_old_files_array_compiled() would already catch files with the exact same name, but this test will also catch files with new names, just to be extra cautious.
Props iandunn, ocean90, SergeyBiryukov
Fixes 43101