CBQE-0: add allocator stats in test_items_append

Change-Id: I2a9c19fdc55db6b4137eee8bcdb0ec20cd35564b
Reviewed-on: http://review.couchbase.org/32730
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>