Functional testing without using simpletest I have recently joined a Drupal project at work that is approx 50% finished.  Functional testing was never part of the project. The site has about 8 custom modules and they are not going to be used anywhere but with this client. I tried implementing the testing module in D7 but the modules were not created in a way that allows the modules to install cleanly. I tried to get them installable but I don't think there will be enough time to devote. 

So, I need a solution that will allow me to test the new features we are building without having to fix the previously built portions.  

I looked at Selenium but to my knowledge it requires the feature to be built before the script can be recorded. I'd like to use it Test Driven Development style. I have also been looking at Behat but honestly, I don't get it.

This question is more specific than "what is your favorite functional testing framework?". It is more specifically, "What functional testing framework works well with Drupal and works with our site as it is currently developed?".
I ended up working with Behat (http://docs.behat.org/en/latest/) and Mink for Behat (http://mink.behat.org/en/latest/) to write tests that emulated a user using a browser on my website.

However, I still needed Drupal integration for quick access to nodes, users, and roles.  I found this project (https://www.drupal.org/project/drupalextension) which incorporates Behat, Mink, and the Drupal API.  This allows me to write tests like this:

@api
Scenario: Logged out user can access the custom Landowner registration page
    Given I am not logged in
        And I am on the homepage
    When I click "Register here"
        And I am on "/register"
        And I click "Register as a Landowner"
        And I am on "/register/landowner"
    Then I should see "Landowner Registration"


Now I can test a Drupal site without the issues involved with Simpletest.