MDL-43738 behat: Backporting new 2.7 methods to 2.5

We encourage backporting new feature files, adding
here also these Moodle 2.7 new step definitions so
future features will be easier to backport as we
would be able to directly cherry-pick them.
MDL-43738 behat: Backport to 2.5

Other issues this patch depends on were not
backported to 2.5 so we need to cover it:
- Add hardcoded waits as MDL-42625 was not
  integrated
- Add behat_form_field::get_internal_field_id()
- Use document.readyState rather than our
  custom JS query (MDL-42625)