Reduce the amount of logging produced by *RenegotiateMany* tests

The RenegotiateMany* tests repeat testing that's done by other tests,
only multiple times. This causes the log output to be excessive.
This CL raises the output warning level to `LS_WARNING` for those tests.

Example failure of RenegotiateManyVideoTransceiversAndWatchAudioDelay:
https://chromium-swarm.appspot.com/task?id=73f4ebaffd42cd11&o=true&w=true

Number of log lines: 15570
With this CL: 19

Bug: none
Change-Id: I781d32097d911b411c86fdc86564dd9413b70f5d
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/416440
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#45916}