Behat form fields are implemented in a way completely isolated from the
rest of the Behat Context system. Whereas regular step definitions have
access to execute steps, to call `find`, check for JS running, and other
related functionality, the Moodle implementation of a field type does
not have any access to this.
By moving the core functionality of behat_base to a new trait, and the
constants to a new interface, the functionality can also be used in
behat form fields in the same way as elsewhere.
Andrew Nicols