SyncFakeServerTestCase: setup/teardown cleanup

Before this CL, the setup and teardown logic in SyncFakeServerTestCase
was a bit inconsistent:
A bunch of cleanup was done in both setUp and tearDown (redundant),
but clearBookmarks was only called in tearDown (inconsistent).

After this CL, the same cleanup is done once in setUpForTestCase, i.e.
before any of the tests in this file run, and then once in tearDown,
i.e. after each individual test.

Bug: 1486420
Change-Id: Iaa460c2814959972b558aed561dc6d9843318c02
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5130054
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1238586}