Test alt to toggling a11y attributes of conditional

To fix "Elements must only use allowed ARIA attributes"
#979

Attempted solution:
- Remove aria-expanded
- Follow "Optionally, each element that serves as a container for
panel content has role region and aria-labelledby with a value that
refers to the button that controls display of the panel" from
https://www.w3.org/TR/wai-aria-practices/#accordion

src/govuk/components/radios/radios.js

@@ -41,8 +41,6 @@ Radios.prototype.setAttributes = function ($input) {
   if ($content && $content.classList.contains('govuk-radios__conditional')) {
     var inputIsChecked = $input.checked
 
-    $input.setAttribute('aria-expanded', inputIsChecked)
-
     $content.classList.toggle('govuk-radios__conditional--hidden', !inputIsChecked)
   }
 }

src/govuk/components/radios/template.njk

@@ -90,7 +90,8 @@
           {% endif %}
         </div>
         {% if item.conditional %}
-          <div class="govuk-radios__conditional{% if not item.checked %} govuk-radios__conditional--hidden{% endif %}" id="{{ conditionalId }}">
+          <div class="govuk-radios__conditional{% if not item.checked %} govuk-radios__conditional--hidden{% endif %}" id="{{ conditionalId }}"
+          aria-labelledby="{{ id }}" role="region">
             {{ item.conditional.html | safe }}
           </div>
         {% endif %}