Fix fragile explicit cert date tests.

Fix fragile explicit cert date tests.

The tests used localtime to format "today's" date, but then extracted a
GMT date from the cert.  The comparison breaks when run late in the
evening west of UTC, or early in the AM hours east of UTC.

Also took care of case when test runs at stroke of midnight, by
accepting either the "today" before the cert creation, or the
"today" after, should they be different.

Fixes fragile tests in #21716

Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/24139)