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}