Adds test coverage for the following mitigations (need both to pass): 1. https://webrtc-review.googlesource.com/c/src/+/375847 2. https://webrtc-review.googlesource.com/c/src/+/376022 Like the comment says, this is neither mandated by the spec or guaranteed, but when the number of codecs is quite small like it is in this test it will be true for all RTX codecs. Bug: webrtc:360058654 Change-Id: Ib73cea59d06a62390dd039eb2dc04677d6178460 Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/375865 Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org> Commit-Queue: Henrik Boström <hbos@webrtc.org> Cr-Commit-Position: refs/heads/main@{#43841}