SSL test framework: port SNI tests

Observe that the old tests were partly ill-defined:
setting sn_server1 but not sn_server2 in ssltest_old.c does not enable
the SNI callback.

Fix this, and also explicitly test both flavours of SNI mismatch (ignore
/ fatal alert). Tests still pass.

Reviewed-by: Rich Salz <rsalz@openssl.org>