Bring some ProfileSyncService unit tests closer to reality

No behavior is changed, only test code.
- Sign-in before creating the sync service.
- Use MANUAL_START, which is more common than AUTO_START these days.

Bug: None
Change-Id: I739b950b97e01321008b72c984b909ec09bde6ee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2540595
Commit-Queue: Victor Vianna <victorvianna@google.com>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828139}