more fixes

common/lib/capa/capa/templates/choicegroup.html

 <form class="choicegroup capa_inputtype" id="inputtype_${id}">
     <fieldset>
-      % if response_data['label']:
+      % if response_data and response_data['label']:
         <legend id="${id}-legend" class="response-fieldset-legend field-group-hd">${response_data['label']}</legend>
       % else:
         <legend>Question</legend>
@@ -24,7 +24,7 @@
                         correctness = None
                 %>
                 % if correctness and not show_correctness == 'never':
-                  <% label_class += ' choicegroup_${correctness}' %>
+                  <% label_class += ' choicegroup_' + correctness %>
                 % endif
                 % endif
                 class="${label_class}"

common/lib/capa/capa/tests/test_input_templates.py

@@ -231,7 +231,7 @@ class ChoiceGroupTemplateTest(TemplateTestCase):
         for test_conditions in conditions:
             self.context.update(test_conditions)
             xml = self.render_to_xml(self.context)
-            xpath = "//label[@class='choicegroup_correct']"
+            xpath = "//label[contains(@class, 'choicegroup_correct')]"
             self.assert_has_xpath(xml, xpath, self.context)
 
             # Should NOT mark the whole problem
@@ -252,7 +252,7 @@ class ChoiceGroupTemplateTest(TemplateTestCase):
         for test_conditions in conditions:
             self.context.update(test_conditions)
             xml = self.render_to_xml(self.context)
-            xpath = "//label[@class='choicegroup_incorrect']"
+            xpath = "//label[contains(@class, 'choicegroup_incorrect')]"
             self.assert_has_xpath(xml, xpath, self.context)
 
             # Should NOT mark the whole problem