Inject a fake TabGroupSyncService in OptimizationGuideServiceTest

This CL makes sure to inject a fake TabGroupSyncService into
OptimizationGuideServiceTest. As of today, TabGroupSyncService is not
created in unit tests but we will soon change this. And when we do, as
the real TabGroupSyncServiceImpl registers some optimization types, the
OptimizationGuideServiceTest-s start to fail, as they don't assume
another service can register optimization types.

Bug: 409499069
Change-Id: I3e5d821b487204b40673ac6559836edf86e60b6b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6482569
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Auto-Submit: Louis Romero <lpromero@google.com>
Commit-Queue: Louis Romero <lpromero@google.com>
Cr-Commit-Position: refs/heads/main@{#1451041}