[TaglibExtractor|Writer] Fix mimetypes, clean up tests
"audio/opus" has never been a valid mime type name in shared-mime-info, or an alias of a valid mime type. The test only ever worked as the tests and the extractor both used the same bogus name verbatim.
Likewise for "audio/mpeg3", which has never been a valid name or alias in shared-mime-info.
"audio/x-mpeg" is a valid alias, but has never been the name of the type, i.e. there has never been an SMI version which returned "audio/x-mpeg" instead of the actual "audio/mpeg".