This test was using time.strftime which uses the current machine timezone,
but collection is done in UTC timezone, so the test was sometimes failing
because the strftime('%Y-%m-%d') was another day than the same time in
UTC time zone.

This test https://erp5js.nexedi.net/#/test_result_module/20210811-96D6B55F/8
for example had the problem. By using faketime and TZ=Europe/Paris
environment variable, the same problems reproduced locally 100% of times.

The fix is to always use datetime.utcnow().strftime('%Y-%m-%d'), to not
depend on the timezone.