Fix nested checkboxes with single top-level node

Nested checkboxes with a single top-level node
will only have one item in their `items` list.
This is because the other choices are children of
that list item.

This means we need to check the `choices`
attribute, which lists all the checkboxes, to see
if they should be marked as a group (by being
wrapped in a `<fieldset>`) or not.

app/main/forms.py

@@ -588,7 +588,7 @@ class govukCheckboxesField(SelectMultipleField):
         }
 
         # add HTML to group items if more than one
-        if len(items) > 1:
+        if len(field.choices) > 1:
             params.update({
                 "fieldset": {
                     "attributes": {"id": field.name},