Publish fragment-id in the messages that splitmuxsink and splitmuxsrc send, so when they are received out of order (due to async finalization, for example), they can still be identified / ordered correctly. Fix a race in the splitmuxsink unit test where messages might be received out of order Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/7053>