[tests] Fixed failure in test_change_device_clean_templates

After fixing openwisp/netjsonconfig#197
This admin test in the config app was not failing anymore because
the failure case is now handled automatically in netjsonconfig.

Rather than looking for another cases needed to trigger the error
I simulated the error with mocking to ensure the UI will continue
handling any error in such cases.

Author: nemesifier

openwisp_controller/config/tests/test_admin.py

@@ -4,6 +4,7 @@
 
 from django.contrib.admin.models import LogEntry
 from django.contrib.auth import get_user_model
+from django.core.exceptions import ValidationError
 from django.test import TestCase
 from django.urls import reverse
 from swapper import load_model
@@ -489,7 +490,10 @@ def test_change_device_clean_templates(self):
             }
         )
         # ensure it fails with error
-        response = self.client.post(path, params)
+        with patch.object(
+            Config, 'clean_templates', side_effect=ValidationError('test')
+        ):
+            response = self.client.post(path, params)
         self.assertContains(response, 'errors field-templates')
         # remove conflicting template and ensure doesn't error
         params['config-0-templates'] = ''