I added a test called JGroupsSubsystemTest which uses our subsystem testing framework which attempts to validate the description providers, this will be committed to upstream in the next few days.
To make the tests pass I had to override AdditionalInitialization.getModelValidationConfiguration() as follows:
Returning null basically turns off validation of the providers. What I have added is a minimum of what is required for subsystem testing, the test framework allows you to do a lot of other stuff, see the jmx subsystem test for some examples.
The wip lives at https://github.com/kabir/jboss-as/tree/subsystem-test-model-fixes