Adding unit test coverage to for kFledgePst

Follow-up from crash involving missing finch flag check, adding unit
test coverage for the scenario involved.

Bug: 1426996
Change-Id: I4e6cbd8314be2971eab8199216bd57b02117c009
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4381247


Reviewed-by: Maks Orlovich <morlovich@chromium.org>
Commit-Queue: Tim Huber <timhuber@google.com>
Cr-Commit-Position: refs/heads/main@{#1123738}