CBQE-0: add the unit size in logs for test_items_append

Change-Id: I6f763b931be880d5eead1461e155ee3b51251493
Reviewed-on: http://review.couchbase.org/32411
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>