In your test environment ActionMailer::Base.delivery_method should be set to :test, which means that these emails will not be sent out. If this setting is set to something else such as smtp by way of a configuration in say config/environments.rb, then emails will be sent out.
If that setting’s already there, then to use the User object (as in, to be actually able to log in) you’ll need to call confirm! on it:
user = User.first
user.confirm!