Set fixed dates for test certificates validity

Set fixed dates for test certificates validity

Rather than specifying a validity of 10 000 days into the future
during test certificate generation, this hardcodes the notBefore
and notAfter attributes to known values. This will allow writing
tests on the validity of the certificates without knowing when a
specific certificate was regenerated.

This is done as a prerequisite for an upcoming patch which adds
notBefore and notAfter to pg_stat_ssl and sslinfo.

Discussion: https://postgr.es/m/EE288A58-947E-479A-9D99-C46C273D7A23@yesql.se