Simplifies the test so that it only tests the padding-related parts.
Header extensions for padding already has a dedicated test, as does
packet stats from RtpSenderEgress.
Bug: webrtc:11340
Change-Id: I88829409aac15f0aad0d4d634114731e819574bd
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/218844
Commit-Queue: Erik Språng <sprang@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34019}