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This test would hang if anything ever went wrong, because
failures were not being checked.

Rewrote the test to always use failure callbacks.
Also switched to using rtcp-mux.

Tested by 120 runs on workstation.

Bug: chromium:452623
Change-Id: Ie73c32edf22b4ca40e9f5e01b9e9fe5cc054623f
Reviewed-on: https://chromium-review.googlesource.com/967761
Commit-Queue: Harald Alvestrand <hta@chromium.org>
Commit-Queue: Tommi <tommi@chromium.org>
Reviewed-by: Tommi <tommi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543936}