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)