In dbtests.cpp, we upgrade a collection level IX lock to an X lock: https://github.com/10gen/mongo/blob/9b9a63ba1119f69154b360b2758091b64e0310ce/src/mongo/dbtests/dbtests.cpp#L181-L195

Simple fix would be to take the X lock right away but we can also potentially drop the IX lock on the collection before taking the X lock? Either way, if we acquire the IX lock for the collection and go to drop the collection, we would be upgrading the lock so we need to address that.