Remove event waits from VideoStreamEncoder tests

These tests run on simulated time, but Event::Wait() yields only once
to run the tasks that are ready at the current timestamp before
blocking on the wall clock. Anything that needs time to advance is
never run, and the wait becomes a five second stall.
SetSourceAndWaitForFramerateUpdated() ignored the return value, so
such a stall passed unnoticed.

Drain the encoder queue with AdvanceTime(TimeDelta::Zero()) instead.
The Trigger* helpers already did so right after their wait, making
the event pure duplication. CpuOveruseDetectorProxy used its event
only to report whether OnTargetFramerateUpdated() had been called,
which is now a flag. This removes the last blocking waits on the
encoder queue, a prerequisite for merging it into the worker queue.

Bug: webrtc:42222804
Change-Id: Iadfffdd82c3b02b1e82f1d066c45ad762812266a
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/504440
Commit-Queue: Markus Handell <handellm@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48652}