[Fixit][CodeHealth] Move pref registration for testing

This CL moves the pref registration for commerce testing out of
test_utils so that classes like account_checker and depend on it without
introducing circular dependency.

This CL is needed in the follow-up CL https://crrev.com/c/6099066, and
also make it easier for future tests to register prefs.

Bug: 383137544
Change-Id: I2143884333be91896cfd9576d0e9016eac899dc3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6098326
Commit-Queue: Yue Zhang <yuezhanggg@chromium.org>
Owners-Override: Yue Zhang <yuezhanggg@chromium.org>
Reviewed-by: Matthew Jones <mdjones@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1398304}