Added a few tests cases which are exposing some problems centered around connection.close(), and the handling of things on the backends.

I have a sneaking suspicion that we need to properly handle automatic rollback of incomplete TX's when a close() is called.

This is something that c3p0 handled... However, I'm not -quite- sure how it would apply here, since the XA pooled connection logic did some trickery on this (isXA is used for this, iirc.)

Instead of fixing these tests, I'd rather see a new suite for our XA driver, where we have tests that open / close connections, use multiple xids, etc. and then verify how many connections are actually held / closed / etc. in the XADataSource implementation.
Once we prove those things work, then it would make sense to move on to writing more tests covering corner-cases and fuzzy spots where the specs aren't very clear. (mixing local and XA transactions, using a Connection handle in multiple threads, etc....)