CBQE-1514: fix  MemorySanity.test_items_append

Change-Id: I6fa00bd9ae7e01aa2c6fc1b8cf9d696f5c15950f
Reviewed-on: http://review.couchbase.org/28345
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>