MDL-40666 enrol|cohort: PARAM_NUMBER -> PARAM_INT

PARAM_NUMBER is a float, our id's are int's

* Fix core_cohort_get_cohorts to specify correct return type on cohort id
* Fix core_enrol_get_enrolled_users_with_capability to specify correct
  correct return type on user id
* Fix core_cohort_update_cohorts to specify the right parameter type
  (int) on cohort id
* Add tests to verify the bug on core_cohort_update_cohorts param type.