CASSANDRA-7423 Test

@knifewine for review. No rush. Just a basic test that the UDT subfield update syntax works, non-frozen UDT's can be used, and LWW is working within UDT's.

Let me know how you feel about the comments in the new test.
I assumed it would be invalid for the same reason the prior query would be invalid. As to whether those will actually be supported, I don't know and probably won't until some development happens on the ticket. I will clarify on the ticket before merging this,