[tests] Updated failing api test cases in connection

As we are now restricting the choices at serializer
level, the checks in api test cases required a small
change to handle this.

Signed-off-by: DragnEmperor <dragnemperor@gmail.com>

Author: DragnEmperor

openwisp_controller/connection/tests/test_api.py

@@ -338,8 +338,8 @@ def test_non_existent_command(self):
             )
             self.assertEqual(response.status_code, 400)
             self.assertIn(
-                '"custom" command is not available for this organization',
-                response.data['input'][0],
+                '"custom" is not a valid choice.',
+                json.loads(response.content)['type'][0],
             )