MDL-43837 behat: Functional tests for fields management

We are cheating with this .feature file as this
is not a behaviour and we are just testing the
framework, but in this case I think that everybody
will agree on the need of doing it, as every browser
behaves differently and minor changes in the getters
and setters can easily end up breaking the behaviours.
MDL-43738: Normalization and major refactoring of getters and setters

Every single step that sets or gets a value from a field
has been updated to follow the same behaviour both when
using it through a single step or through generic steps
like "I fill the moodle form with:", to resume all the
changes:
- Created a behat_form_group to re-guess the field type
  and act appropriately setting and getting it's value
- Normalize all getters and setters to use behat_form_field
  children
- Complete behat_form_checkbox to trigger the appropiate JS
  event needed to perform some JS scripts that are listening
- Refactor MDL-43713 multi-select changes and remove
  the two new steps introduced there as that behaviour can
  be managed from the generic getter