Add the ability to include additional Julia registries

Features added

• The get_responder function now takes an additional registry_urls argument, which can be used to define additional Julia registries to include in the docker build.

Specific changes

• Added registry_urls to get_responder
• Changed jottest2 and jottest4 to both use a package defined in an external registry, as an automated test
• Refactored test code to make it easier to call specific quartet tests (eg 2 and 4 only).