Migrate unit tests to use CreateTestEnvironment

This CL performs a repository-wide migration of unit and integration
tests from the production CreateEnvironment() factory to the test-only
CreateTestEnvironment() factory.

The production factory defaults to no field trials, and ignores
command-line overrides (like --force-fieldtrials) when used
within test fixtures. CreateTestEnvironment() ensures that tests
correctly inherit and propagate the global test environment state,
enabling more reliable field trial testing and supporting the
ongoing payload type allocation redesign.

Bug: webrtc:515090586
Change-Id: I4ae9c0259e6547e569bd9aa70c338174ac30370b
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/474380
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47758}