[iOS][Credential-Provider] Add fieldtrial test config

Bug: 1223936
Change-Id: Ic66503625d06bbc26f0516717b63e539f86f11ab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3139024


Auto-Submit: Javier Flores <javierrobles@chromium.org>
Commit-Queue: Robbie Gibson <rkgibson@google.com>
Reviewed-by: Robbie Gibson <rkgibson@google.com>
Cr-Commit-Position: refs/heads/main@{#918770}