Unbreak test suite.

Updates CoreDataHelpers to call ATHNotify() on root context save.
Updates methods to use AccountService instead of WPAccount where needed.
Removes some obsolete code involving background contexts.
Updated Core Data test helper to use background context as root
Stub out call for OAuth2 token
Deleted objects retrieved by ID with objectById always fault

Use existingObjectWithId to get nil when it doesn't exist.
Possible fix for oddities when things save
Removing the unnecessary notification object check
Added missing unregister to reset of test helper
Merge pull request #1614 from wordpress-mobile/issues/1613-fix-tests

Unbreak test suite.