CBQE-927: increase the number of items for upgrade test

Change-Id: Ibda81214a3370b321284e552546eea3f99a55421
Reviewed-on: http://review.couchbase.org/24852
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>