Add -s option to engine_testapp to spinlock

Change-Id: I98c1f0c5fa5cf85c6162e0ce633802b90549a4f2
Reviewed-on: http://review.couchbase.org/31672
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>