CBQE-0: better logging in test_items_append

Change-Id: I99764ca98810f38db720d03c77e3bcad6bd536cb
Reviewed-on: http://review.couchbase.org/31118
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>