cert-checker: allow 1s of slop in validity periods (#5471)
cert-checker: allow 1s of slop in validity periods (#5471)

We are in the process of reducing our validity periods by one second,
from 90 days plus one second, to exactly 90 days. This change causes
cert-checker to be comfortable with certificates that have either of
those validity periods.

Future work is necessary to make cert-checker much more robust
and configurable, so we don't need changes like this every time we
reduce our validity period.

Part of #5472
jcjones/boulder,letsencrypt/boulder,jsha/boulder,jcjones/boulder,jsha/boulder,jcjones/boulder,jcjones/boulder,letsencrypt/boulder,jsha/boulder,letsencrypt/boulder