[ios][fixit] Use Testing Category instead of Extension

This CL updates some Extensions that should use Category to expose
methods used for tests only.

Extensions should be used for private declarations of properties and
protocol conformance.
Categories, especially +Testing category, should be used to expose
implementation details for tests.

coveraged file

Bug: 1489285
Low-Coverage-Reason: This CL doesn't change any logic in the low
Change-Id: I4c49ccef8612db0b8d160ba5c466101cc8386e0e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5067375
Reviewed-by: Mark Cogan <marq@chromium.org>
Reviewed-by: Federica Germinario <fedegermi@google.com>
Commit-Queue: Asami Doi <asamidoi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1230658}