Reduce to testing what RTPSender is actually interested in: that
packets are actually forwarded to the pacer.
Partially the old test was verifying TransmissionOffset header extension,
add an explicit test for that at RtpRtcp-level instead.
Bug: webrtc:11340
Change-Id: I62be39e1d9d8c214c3277f4f1326db05b937674a
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/218845
Commit-Queue: Erik Språng <sprang@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34023}
726b0e824b