Add UPR step function to engine_testapp api

Change-Id: I3dc5e1ae4d860d0f9f9496932c97de53efb97278
Reviewed-on: http://review.couchbase.org/30247
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>