Fix no-dtls1_2

dtlstest.c needs some adjusting to handle no-dtls1_2 since commit
7bf2e4d7 banned DTLSv1 at the default security level - causing the
test to fail.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Todd Short <todd.short@me.com>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/18848)