Cleanup RtcEventLogCircularBufferTest to rely on propagated clock

Use SimulatedClock for most of the log times,
Replace clock for verification with running current time variable, set
the expected time to the expected event explicitly instead of through
global time and constructor.

While at it, cleanup the test in general: remove fixture state,
remove extra block in favor of deleting log explicitly.

Bug: webrtc:42223992
Change-Id: Iebb571789a794b089886caf0bc7d8695bccf310f
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/419780
Reviewed-by: Björn Terelius <terelius@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46049}