Also expands integration_test_helpers to deal with multiple datachannels. The bug has not yet been triggered. Bug: webrtc:13668 Change-Id: I82a0fdae0cc32815c250a691b56c614bfd6d606b Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/251602 Reviewed-by: Henrik Boström <hbos@webrtc.org> Commit-Queue: Harald Alvestrand <hta@webrtc.org> Cr-Commit-Position: refs/heads/main@{#35980}