@SuppressWarnings("unchecked")
    public static <T extends ConfigInstance> Class<? extends ConfigInstance.Builder> getBuilder(Class<T> configClass) {
        for (Class<?> memberClass : configClass.getClasses()) {
            if (memberClass.getSimpleName().equals("Builder"))
                return (Class<? extends ConfigInstance.Builder>) memberClass;
        }
        throw new RuntimeException("Missing builder");
    }
Use for testing. Use as parent for the config producer(s) you want to test, to test
only a subtree of the producers.

@author gjoranv
config-model/src/main/java/com/yahoo/config/model/test/MockRoot.java