LibreSSL 3.2.4 made the certificate verification logic back closer to
pre-3.2.2 one, which is more compatible with OpenSSL.

Part of the fixes added by commit a0e98d48c91f ("Enhance TLS 1.3 support
on LibreSSL 3.2/3.3", 2020-12-03) is required for 3.2.2 and 3.2.3 only
(and ~3.3.1, however 3.3 does not have a stable release yet). Since both
releases are security fix, it should be safe to remove those special
treatment from our test suite.

While we are at it, TestSSL#test_ecdh_curves is split into TLS 1.2 and
TLS 1.3 variants for clarity.

https://github.com/ruby/openssl/commit/a9954bac22