The enhancer has never handled jarred classes very well.
If we rely on the enhancer my guess would be that doing a top down build would work (maven operates on target/test-classes in this case), but building just the module will fail (maven picks up the jar from .m2/repository). I'd expect the same behavior for test-jar or a separate module though.
Maybe I need to read more on the validation aspect of JPA 2.0. Are we relying on the entities having been enhanced, or do we need to re-enhance them? If it's the former we can probably reorder the enhancement to occur before test-jar does its work. For the latter we'll have to unjar or improve the enhancer to handle jarred classes.