Update previewPane JS and JS radios test helpers

The previewPane JS used selectors that targeted
the old form of radios HTML.

The JS tests also contained selectors like this
and fragments of HTML, used for fixtures, modelled
on the old radios HTML.

app/assets/javascripts/previewPane.js

@@ -4,7 +4,7 @@
 
   $ = global.jQuery;
 
-  let branding_style = $('.multiple-choice input[name="branding_style"]:checked');
+  let branding_style = $('.govuk-radios__item input[name="branding_style"]:checked');
 
   if (!branding_style.length) { return; }