tests: clean up bad extension

The presence of the "babar" extension breaks subsequent tests.
So delete the file and create an empty one to return the config
to sanity.

tests/test-check-py3-commands.t

@@ -65,6 +65,9 b' out some potential warnings that come fr'
   *** failed to import extension babar from imaginary_elephant: *: 'imaginary_elephant' (glob)
   warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
 
+  $ rm included-hgrc
+  $ touch included-hgrc
+
 Test bytes-ness of policy.policy with HGMODULEPOLICY
 
   $ HGMODULEPOLICY=py