Add "learn how to test this patch" on project-issue followups

Inspired by this: http://www.drupal4hu.com/node/137

When there's an attachment on a project issue followup, it might help get more reviewers to have a link off to http://drupal.org/patch/apply and/or http://drupal.org/patch/review right there next to the upload attachment. Seems to already be code identifying which is the most recent comment with an attachment, so it'd be a matter of theming a link in on the comment itself according to the same logic.

Something like "Learn how to test this patch".