You can run the following from the root directory of ActiveMerchant:
ruby test/remote/gateways/remote_authorize_net_test.rb
Replace with the gateway you want to run the tests for.
--
Cody Fauser
http://shopify.com - e-commerce done right
http://www.codyfauser.com - blog
http://peepcode.com/products/activemerchant-pdf - ActiveMerchant PeepCode
http://www.oreilly.com/catalog/rjsrails - RJS Templates for Rails