sslutil: set context security level for legacy tls testing (issue6760)
Current versions of OpenSSL do not allow the use of TLS <1.2 when the
library's security level is >=1 (1 being the default on most distributions).
Setting the security level in addition to the minimum protocol is therefore
necessary for the legacy protocol tests.
This is done here ONLY when testing, when:
- explicitly setting the cipher string, or
- using the "--insecure" flag, or
- using the "devel.serverexactprotocol" testing option.
See: https://github.com/openssl/openssl/blob/master/NEWS.md#major-changes-between-openssl-30-and-openssl-310-14-mar-2023