add test-case to verify `error.path` when a helper was not found.

This verifies the code added with:
99c9d18601

actionpack/test/abstract/helper_test.rb

@@ -116,6 +116,11 @@ module AbstractController
         e = assert_raise(AbstractController::Helpers::MissingHelperError) { AbstractInvalidHelpers.helper(:missing) }
         assert_equal "Missing helper file helpers/missing_helper.rb", e.message
       end
+
+      def test_missing_helper_error_has_the_right_path
+        e = assert_raise(AbstractController::Helpers::MissingHelperError) { AbstractInvalidHelpers.helper(:missing) }
+        assert_equal "helpers/missing_helper.rb", e.path
+      end
     end
   end
 end