Add test checking that any variable declared with no default value will generate a proper error message.

testing/examples/babel.org

@@ -307,3 +307,16 @@ src_sh{echo 2} blocks on the src_emacs-lisp{"same"} line
 
 #+results:
 [[file:./cv.cls]]
+
+* no default value for vars
+  :PROPERTIES:
+  :ID:       f2df5ba6-75fa-4e6b-8441-65ed84963627
+  :END:
+
+There is no default value assigned to =x= variable. This is not permitted
+anymore.
+
+#+source: carre(x)
+#+begin_src python
+  return x*x
+#+end_src