CBQE-0: slightly decrease num items in test_items_append

to avoid memory overload:
http://qa.hq.northscale.net/job/centos_x64--51_01--mem_sanity-P1/11/console

Change-Id: Icfce5c982e31a2e8a0cb120aff66a7054c733beb
Reviewed-on: http://review.couchbase.org/37845
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>