x509asn1: unittests and fixes for gtime2str

x509asn1: unittests and fixes for gtime2str

Fix issues in GTime2str() and add unit test cases to verify correct
behaviour.

Follow-up to 3c914bc6801

Closes #14316

Backported to Debian by Carlos Henrique Lima Melara <charles@debian.org>.

Changes:
- In this version, GTime2str doesn't return CURLcode, so change that to NULL.
- Also change test helper function to match the correct type and pass the
  correct arguments. In this version, GTime2str doesn't take struct dynbuf *.
  It's aimed to not FTBFS if someone build the package with --enable-debug.

Gbp-Pq: Name CVE-2024-7264-1.patch