Hi Rex,
Good to hear that
What you can do with the current step definitions is to follow the required step to create a new role, this would be useful if you would just need to test a couple of features related with your custom role, but I guess that you, like many other organizations using custom roles, would base most of your scenarios on these roles so you need a quicker way to set them up; if you are interested in always having the same custom roles in each scenario you can create a new step Given I add rex custom roles or even more generic Given I add rex data set in local/rexrules/tests/behat/behat_local_rexrules.php and use PHP code to generate all you need, using testing data generators or direct Moodle APIs calls, then each feature requiring these datasets can include it. Note that this approach (to use PHP code to set data) goes against the black-box principle of acceptance testing so, at a practical level, you should restrict the use of this steps to the Given section (settingthe scenario contexts).
An addition, what I would be happy to include in core would be a data generator for create roles, as I guess more organizations would be interested in using it. (https://tracker.moodle.org/browse/MDL-44108)
Regards,
David