Fix ffmpeg/libav test failures and test in CI
If we are going to carry this backend we need to test it! :-)
Fixes #54
This is now ready for review. The ffmpeg/libav backend now has same feature set as GStreamer extractor apart from:
- replay gain metadata (we don't have tests for this)
- CUE sheet extraction (see #230)
The writeback-audio MP4 test is removed in this branch because: (1) it was testing writeback via XMP metadata, not M4a tags; and (2) it seems impossible to get the necessary avdec_aac element to work in latest Fedora 36.