Added "engine-tests" as a separate make target

Change-Id: I8aa04fdb5b61b10a2a986f688bc95a836ce23c41
Reviewed-on: http://review.couchbase.org/30653
Tested-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>