ssl-opt.sh: Add tests checking that MFL implies bounds on MTU
ssl-opt.sh: Add tests checking that MFL implies bounds on MTU

This commit introduces some tests to ssl-opt.sh checking that
setting the MFL limits the MTU to MFL + { Maximum Record Expansion }.

ARMmbed/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls