Add test helper to inject fake media engine for those tests.
Bug: webrtc:15574
Change-Id: Iae4282d2d3b9804548ccadf58797f39508f07c6a
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/325880
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#41075}