How to tell selenium to use test database?
You need to point Selenium to a site that uses the test databases. Selenium talks to your rails, not the database. So, you need to point Selenium to a web app whose config points to your test db, not the dev db.
Hope that helps.
You need to point the Selenium test at an instance of your app that uses the test database.
I wrote a separate deploy task in my capistrano deploy.rb that deploys the application to a testing server, points it at the test db, and loads the selenium-specific fixtures. Selenium treats a deployed app as a black box, so it's up to you to fit all that in to your existing deployment setup.