Within the test/ directory you can run bundle exec compass watch while you're making changes to Foundation and want to see them reflected on-the-fly.
If you're adding a new feature to Foundation you can either update one of the existing test/*.html files or copy test/template.html to a new file. Then make sure you commit any changes to the test pages with your pull request.
NOTE: If you add a new page make sure to include it in the available testing pages list on this page.