The extracts and refactors some test code from rtp_sender_unittest.cc
and puts it in a new target intended to only test RtpSenderEgress, and
do it as pure unit test, rather than the unholy
not-quite-unit-not-quite-integration-test thingy we have today.
Only a first test case is actually ported with this CL, but it's a
start...
Bug: webrtc:11340
Change-Id: Ie2cdde63a00a6ff6eba7b8d443eeb76ce2a527c9
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/216180
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33841}
