Test that SSL_get_servername returns what we expect

Test that SSL_get_servername returns what we expect

Test this on both the client and the server after a normal handshake,
and after a resumption handshake. We also test what happens if an
inconsistent SNI is set between the original handshake and the resumption
handshake. Finally all of this is also tested in TLSv1.2 and TLSv1.3.

Reviewed-by: Ben Kaduk <kaduk@mit.edu>
(Merged from https://github.com/openssl/openssl/pull/10018)