test(notifications): add conformance tests for the notification system

As part of Refactor registry notifications system (&8628 - closed) we want to make sure that we have some level of testing with the current notification system. This will allow us to start refactoring the code and ensure the functionality is preserved after the refactoring has been completed.

Proposal

• Add a mock server that can receive and validate the notification payload sent by the registry.
• Enable the notification system in a few conformance tests for the supported operations operations as seen in notifications/listener.go :
  • ManifestPushed
  • ManifestPulled
  • ManifestDeleted
  • BlobPushed
  • BlobPulled
  • BlobMounted
  • BlobDeleted
  • TagDeleted
  • RepoDeleted