Stop using time(NULL) for certificate tests.

Stop using time(NULL) for certificate tests.

The canned testing certificates added in order to fix #41041
will start to expire in a couple of months;
to avoid a test failure then, we should only validate
them against a time when they are valid.

Previously, we got away with using time(NULL) because the old
canned certificate (taken from testing.torproject.org)
was not only signed using SHA-1: it was valid until 2043!