[iOS] Add fieldtrial testing config for Best Features FRE Screen

This CL adds the testing config entry for feature
kBestFeaturesScreenInFirstRunExperience and updates the existing First
Run test cases to accommodate the Best Features Screen.

EGTests for the Best Features screen will be added in a follow up CL.

Fixed: 406787615
Change-Id: I57c06985d85dec0e9ba2eb3cb9ffbdeefc6f3180
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6408437
Reviewed-by: Jérôme Lebel <jlebel@chromium.org>
Commit-Queue: Hira Mahmood <hiramahmood@google.com>
Cr-Commit-Position: refs/heads/main@{#1440415}