Mark slow tests with @attr('slow')

This removes the ad-hoc 'slowTest()' method. This also allows nose to
exclude execution by using nose -a '!slow'.

See http://nose.readthedocs.org/en/latest/plugins/attrib.html

Change-Id: I9ef291888b10b1d6cc7853653ee2cdce44384244
Reviewed-on: http://review.couchbase.org/36364
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>