Test+fix handling "wrong" downgrade signals

Test+fix handling "wrong" downgrade signals

This accounts for cases that can only occur when een non-compliant server sends
the wrong downgrade signal. (TLS1.1 signal when negotiating TLS1.2 or TLS1.2
signal when negotiating TLS1.0/TLS1.1). According to the TLS1.3 RFC these
cases should be rejected:

RFC8446, section 4.1.3: TLS 1.3 clients receiving a ServerHello indicating
TLS 1.2 or below MUST check that the last 8 bytes are not equal to either of
these values. TLS 1.2 clients SHOULD also check that the last 8 bytes are
not equal to the second value if the ServerHello indicates TLS 1.1 or below.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/27518)