The rate-mismatch regression greps the audio diagnostics for the literal
'MAT/IEC 61937 carrier', but 658469f9 reworded the sink's engage message to
'TrueHD (MAT) via IEC 61937 carrier' when DTS-HD joined the carrier. The test
self-skips on carrier-less CI emulators, so the rot only surfaces on real
hardware, where the fallback behaves correctly and the assertion still fails.
Match the shared 'via IEC 61937 carrier' fragment both codecs emit.
edde746
7cfd68c491