Test ssl_server with both TLS 1.2 and TLS 1.3.
Test against both OpenSSL and GnuTLS.
Clean up compile-time requirements in ssl_server.c: any certificate-based
key exchange is ok, so don't insist on built-in RSA.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>