Is the separate "TestImpl" source file required (e.g. org.jboss.test.as.modular.demos.impl.DemosTestImpl)? It seems like ModularJUnitRunner uses DemosTestCase as the source of the @TestSetup annotation, loads a class from the module it creates, and passes that class to its superclass as the class to test. As long as the module ModularJUnitRunner creates doesn't end up loading that class from the app classloader, it's a separate class. So you shouldn't need a separate source file for the actual test implementation.
I'm going to play a bit and see.