Investigate and fix flakiness of test_final_statistics
It seems that FinalStatistics.request_avg_failed_duration can be zero in some cases during tests even though there are some failed requests.
Example fail:
E assert datetime.timedelta(0) > datetime.timedelta(0)
E + where datetime.timedelta(0) = FinalStatistics(requests_finished=45, requests_failed=5, retry_histogram=[25, 16, 9], request_avg_failed_duration=datetime.timedelta(0), request_avg_finished_duration=datetime.timedelta(microseconds=7503), requests_finished_per_minute=4428, requests_failed_per_minute=491, request_total_duration=datetime.timedelta(microseconds=337613), requests_total=50, crawler_runtime=datetime.timedelta(microseconds=609809)).request_avg_failed_duration
E + and datetime.timedelta(0) = timedelta()```