Add a test for SSLv2 configuration

SSLv2 should be off by default. You can only turn it on if you have called
SSL_CTX_clear_options(SSL_OP_NO_SSLv2) or
SSL_clear_options(SSL_OP_NO_SSLv2). You should not be able to inadvertantly
turn it on again via SSL_CONF without having done that first.

Reviewed-by: Emilia Käsper <emilia@openssl.org>