Fix radio show/hide behaviour outside a form (#375)

The show/hide JS assumes radio buttons are inside a form, as the default behaviour of radio buttons is to scope their name to the nearest enclosing form. If they’re outside a form the JS fails. We can alter it to just select every applicable radio on the page in the absence of a form ancestor.

Adding tests for this involved quite a bit of rework as it turns out that Jasmine will just run every beforeEach() in order rather than letting beforeEach()s lower in scope override the parents.

javascripts/govuk/show-hide-content.js

@@ -80,7 +80,8 @@
     function handleRadioContent ($control, $content) {
       // All radios in this group which control content
       var selector = selectors.radio + '[name=' + escapeElementName($control.attr('name')) + '][aria-controls]'
-      var $radios = $control.closest('form').find(selector)
+      var $form = $control.closest('form')
+      var $radios = $form.length ? $form.find(selector) : $(selector)
 
       // Hide content for radios in group
       $radios.each(function () {