sync: Refactor bookmarks integration tests
Updates the bookmarks integration test helpers to use BookmarkObserver,
rather than the ProfileSyncServiceObserver-based status checking hooks.
Refactors the two-client bookmark integration tests to use this new
checker functionality instead of the old AwaitQuiescence() and
AwaitGroupSyncCycleComplete() calls.
Replaces a python test server legacy test with the help of the
fake-server's new sync entity injection feature.
Deletes a few enable/disable tests.
BUG=
97780,
95742