[tests] Flaky test test_devicegroup_list_api

[nemesifier]

FAIL: test_devicegroup_list_api (openwisp2.sample_config.tests.TestConfigApi) [should not contain default or required templates]
  (0.53s) test_ip_not_in_add_device
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Traceback (most recent call last):
  File "/home/runner/work/openwisp-controller/openwisp-controller/openwisp_controller/config/tests/test_api.py", line 687, in test_devicegroup_list_api
    self.assertNotContains(r, t2.name)
  File "/opt/hostedtoolcache/Python/3.8.13/x64/lib/python3.8/site-packages/django/test/testcases.py", line 481, in assertNotContains
    self.assertEqual(real_count, 0, msg_prefix + "Response should not contain %s" % text_repr)
AssertionError: 2 != 0 : Response should not contain 't2'