[bug:tests] Flaky test: TestWifiClient.test_store_result

[nemesifier]

Another flaky test is showing up randomly in the CI build:

FAIL: test_store_result (openwisp_monitoring.check.tests.test_wifi_client.TestWifiClient)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/runner/work/openwisp-monitoring/openwisp-monitoring/openwisp_monitoring/check/tests/test_wifi_client.py", line 63, in test_store_result
    self.assertEqual(result, {'wifi_clients_min': 3, 'wifi_clients_max': 3})
AssertionError: {'wifi_clients_min': 0, 'wifi_clients_max': 3} != {'wifi_clients_min': 3, 'wifi_clients_max': 3}
- {'wifi_clients_max': 3, 'wifi_clients_min': 0}
?                                             ^

+ {'wifi_clients_max': 3, 'wifi_clients_min': 3}
?                                             ^