Allow option to skip slow tests

Change-Id: Id2e07dd8b68cee407e4e3154c3787712ac60ce2f
Reviewed-on: http://review.couchbase.org/26298
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Reviewed-by: Mordechai Nunberg <mnunberg@haskalah.org>
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>