Add bidirectional audio test and improve SCReAM test coverage

1. Refactor SendMediaInOneDirection to SendMedia to support bidirectional
   media and optional adaptive ptime.
2. Add CreateNetworkPathWithVaryingCapacity helper.
3. Add AvailableSendBitrateIsZeroOrBetween matcher.
4. Add AudioOnlyBothDirectionsLinkCapacity40To90KbpsRtt200ms test.
5. Increase accepted BWE upper bound in LowBweOnLinkWith5PercentUniformLoss
   to avoid test flakiness.
6. Update SendVideoOnlyReturnLinkWithBurstLoss to handle temporary zero
   BWE when feedback packets are lost and encoder pauses.

Bug: webrtc:447037083
Change-Id: I4c9613f83212422c6f012ea4eadf5a4e7b170669
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/506920
Reviewed-by: Björn Terelius <terelius@webrtc.org>
Commit-Queue: Per Kjellander <perkj@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48776}