Define GOVSPEAK_FIELDS only on models that have it

[this safe_html_validator_test](https://github.com/alphagov/govuk_content_models/blob/master/test/validators/safe_html_validator_test.rb#L68-L81) mandates all models to validate using the `SafeHtml` validator irrespective of whether they have any govspeak or not. this in turn requires all models to define `GOVSPEAK_FIELDS`.

i've updated the test to only validate that models that define `GOVSPEAK_FIELDS` validate using `SafeHtml`. models have better reason to define `GOVSPEAK_FIELDS` in that it helps [content api identify](https://github.com/alphagov/govuk_content_api/blob/2014fda929ca5aa7e717040633892943eaa7b110/lib/presenters/artefact_presenter.rb#L113-L117) which fields need to be rendered as govspeak, so defining it is not likely to get missed.
@alext: i did wonder about what would be the best option, but wasn't sure. thanks for suggesting! fixed.