test_helper.rb

# ... other require statements
require 'authlogic/test_case'

class ActiveSupport::TestCase
  # ... whatever is already in here
  def login_as(user)
    activate_authlogic
    user_session = UserSession.new(:email => user.email, :password => 'asdfasdf')
    user_session.save!
    return false
  end
end

## Then in your test...

class UsersControllerTest # etc

  setup do
    @user = # make a user with User.new, factory, etc
    login_as(@user)
  end
end