sunfish: Add unit tests for when feature is disabled

This ensures that we do not accidentally expose Sunfish features even if
the feature has been explicitly disabled (perhaps from a Finch
killswitch).

Bug: b:378590440
Change-Id: I6a6a21000e89dbb15d30c1b75fb39310b556f567
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6011641
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Auto-Submit: Michael Cui <mlcui@google.com>
Cr-Commit-Position: refs/heads/main@{#1381808}