use safe_constantize instead constantize in AC::TestCase

actionpack/lib/action_controller/test_case.rb

@@ -352,9 +352,7 @@ def controller_class
         end
 
         def determine_default_controller_class(name)
-          name.sub(/Test$/, '').constantize
-        rescue NameError
-          nil
+          name.sub(/Test$/, '').safe_constantize
         end
 
         def prepare_controller_class(new_class)