Usage of cohort Behat step for adding cohort member should be converted to generator

See: https://github.com/dravek/moodle/blob/1a744030d639f93f2a147d40f0a38572fa0f846d/cohort/tests/behat/behat_cohort.php#L47

This is a somewhat sub-optimal step, a bit strange in its comparison of specific DOM elements before navigating elsewhere

We should replace usage of it outside the cohort subsystem to just use data generators for Behat scenarios, to both improve their runtime and avoid this step altogether

Found while reviewing davidmatamoros's work on MDL-73141 (we should re-examine some of the changes to Behat in that issue at the same time)