Small changes to the unit tests for send side congestion controller.
Mostly adding some extra waits and expectation verifications. This
prepares for an upcoming CL.
Bug: webrtc:8415
Change-Id: Id3086a485eda99732d01192cac9a91141158ab45
Reviewed-on: https://webrtc-review.googlesource.com/59223
Commit-Queue: Sebastian Jansson <srte@webrtc.org>
Reviewed-by: Björn Terelius <terelius@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22270}