test/test_collect: fix some timezone dependent tests

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.