My comapny had this same problem when looking into upgrading to EJB 3.0. We also attempted to use EJB3Unit as you have linked above, however there was a lot of hand holding we had to do in order to get it to 'sort of' work. The project is essentially dead from the last time we looked at it.
A better option we found was OpenEJB. (Link here:http://openejb.apache.org/)
It took a little bit to get up and running, buit once we got it there it works like a champ. We can hit against any database we doploy normally to, and it throws the same exceptions under the same circumstances (from what we have seen) as JBoss does.
If I can help get things set up for you let me know. I will help however possible.