Fix peerconnection_unittests for force-test-env

Update unittests in peerconnection_unittests to use test environment
and test field trials instead of production ones, allowing them
to pass with the --force-test-environment flag.

Specifically, fix a lifetime issue in PeerConnectionTestWrapper
where the field trials unique_ptr was deleted too early by moving it.

Bug: webrtc:521534686
Change-Id: I36004a5574896fb0fa0c487d2417d8f22f080930
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/480881
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47975}