Test consistency between default.php and metadata.php

Add a test to ensure that every conf['...'] entry in conf/default.php has a corresponding meta['...'] entry in metadata.php.
Keys enclosed in double quotes are possible as well, isn't? Can you handle that case as well?

Maybe an idea to just include the file? (but not as global, because that can conflict with the global `$conf` array.)
That way you handle the keys as real php strings, which is equal to the real use case.