if u used GetRegistry()->Register instead of  ManifestHandlerRegistry::Register everywhere else, u wouldn't need ScopedTestingManifestHandlerRegistry and all this mess with singleton overriding, so please:
1)  Remove all static methods (clients shell access default registry via access to a singleton ManifestHandlerRegistry object)
2) Remove 'SetForTesting' api
3) Just create a new ManifestHandlerRegistry instance for testing and test it.