Test an old style PSK callback with no cert will prefer SHA-256

If using an old style PSK callback and no certificate is configured for
the server, we should prefer ciphersuites based on SHA-256, because that
is the default hash for those callbacks as specified in the TLSv1.3 spec.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6215)