Thanks for asking because there were different projects, things changes fast, and it can be confusing. The main documentation page always up-to-date with the dev. status is http://docs.moodle.org/dev/Acceptance_testing
- moodle-behat-features: Was the first approach, you can use the steps provided by Behat and other libraries, don't take it seriously please, use it as a playground, the second one is much better
- http://tracker.moodle.org/browse/MDL-36269: Is the work in progress branch with tests, you can play with it and make an idea of how it works, but at the moment there are not enough steps definitions to test complex features, we are trying hard to make it very easy to use for devs. and for tests writers.
The behat-moodle project was part of the second option, but we found a better way to manage it and now is useless (in fact I've just deleted the project from my github account)
You can use it and all the feedback you can provide will be very appreciated but consider that it's not even integrated in master (future Moodle 2.5) so there can be changes. The main tracker issue is: http://tracker.moodle.org/browse/MDL-35611