Some messages were processed after involved objects were destructed,
a.k.a. 'use after free'.
This CL fixes that by disconnecting signals before fixture destruction,
honoring CreateChannel/DestroyChannel symmetry and following what is
done in similar test cases.
Bug: webrtc:11269
Change-Id: I122aca70a9978b752edc01e5f31583f4425f3624
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/165685
Reviewed-by: Qingsi Wang <qingsi@webrtc.org>
Commit-Queue: Yves Gerey <yvesg@google.com>
Cr-Commit-Position: refs/heads/master@{#30214}
bcea217667