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'm not sure if I understand why this would be invalid.

    "INSERT INTO t (id, v) VALUES (0, {first: 'b', second: 1})"