Use a fake task queue to test the RepeatingTask rather than a real task queue, which removes the need for Sleep(). This fixes the flakiness issues as the class is no deterministic. BUG=webrtc:12808 Change-Id: I8c6a8535165b076f5fe6ec3e65ebcf7f07008737 Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/237803 Reviewed-by: Tommi <tommi@webrtc.org> Commit-Queue: Evan Shrubsole <eshr@webrtc.org> Cr-Commit-Position: refs/heads/main@{#35349}