Use instance_eval instead of adding an accessor to the class

actionpack/test/controller/layout_test.rb

@@ -146,8 +146,7 @@ class LayoutExceptionRaised < ActionController::TestCase
   def test_exception_raised_when_layout_file_not_found
     @controller = SetsNonExistentLayoutFile.new
     get :hello
-    @response.template.class.module_eval { attr_accessor :exception }
-    assert_equal ActionView::MissingTemplate, @response.template.exception.class
+    assert_kind_of ActionView::MissingTemplate, @response.template.instance_eval { @exception }
   end
 end