[Plan B] Remove global deprecation suppression from rtc_test

and apply local markup.

This is part of the transition to ensure all Plan B usage in tests is
explicitly marked.

Modified:
- webrtc.gni: Removed automatic addition of suppress_plan_b_deprecation to rtc_test.
- pc/*_unittests.cc: Added suitable markup
- test/testsupport: Fixed an inappropriately suppressed deprecation

Bug: webrtc:467984837
Change-Id: I18197155d7e521634cd0738357e8504567bbdebd
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/452160
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47003}