Check that checkbox aria-controls is a conditional

Check that the checkbox aria-controls target has the `govuk-checkboxes__conditional` class before trying to toggle the `--hidden` modifier.

This mirrors a similar check that we do for conditional reveals associated with radios, and makes it more robust if e.g. aria-controls is being used for things that are not conditional reveals, now that we're not scoping the aria-controls lookup to the $module.

src/govuk/components/checkboxes/checkboxes.js

@@ -61,7 +61,7 @@ Checkboxes.prototype.setAttributes = function ($input) {
   $input.setAttribute('aria-expanded', inputIsChecked)
 
   var $content = this.$module.querySelector('#' + $input.getAttribute('aria-controls'))
-  if ($content) {
+  if ($content && $content.classList.contains('govuk-checkboxes__conditional')) {
     $content.classList.toggle('govuk-checkboxes__conditional--hidden', !inputIsChecked)
   }
 }