Fix rtc_unittests for force-test-env flag

Update unittests to use CreateTestEnvironment and
CreateTestFieldTrials instead of production versions,
and add bypasses to low-level implementation tests,
allowing rtc_unittests to pass with the
--force-test-environment flag.

Bug: webrtc:521534686
Change-Id: I2c70d254a6950db86bdd129ea690c9fc7b4901ef
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/480221
Auto-Submit: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47962}