Add force-test-environment flag and checks

Add --force-test-environment flag to test_main.
This flag sets a global boolean that CreateEnvironment
and FieldTrials will check, crashing if they are used
in tests instead of the test-specific versions.

Generated by prompt:

We want a way to detect if CreateEnvironment or CreateFieldTrials is
used in tests where we should be using CreateTestEnvironment or
CreateTestFieldTrials. Add code to test/test_main.cc to have a
command line flag '--force-test-environment' that sets a global
boolean that CreateEnvironment and CreateFieldTrials will RTC_CHECK on
so that we immediately crash if the non-test versions are used.

Bug: webrtc:521534686
Change-Id: I7f47b4ccfc396f90d7626d430140e92834b6a483
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/480160
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47947}