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}