We need to reconfigure SubscriptionDefinition and SubscriptionDefinitionRegistry so that it is easier to load test data.  Right now the SubscriptionDefinitionRegistry loads YAML files from the subscription_configs directory.  It's possible to create test YAML files and drop them in there, but it would be nice to also be able to create a SubscriptionDefinition object in an ad hoc way in a test and add it to the registry.  Or even better: to define the YAML in the test with Java's fancy new multiline strings and then pipe that into the registry to get it loaded.