Increase max_frag_len / MTU in fragmentation ref tests
Increase max_frag_len / MTU in fragmentation ref tests

The tests "DTLS fragmenting: none (for reference)" and
"DTLS fragmenting: none (for reference) (MTU)" used a
maximum fragment length resp. MTU value of 2048 which
was meant to be large enough so that fragmentation
of the certificate message would not be necessary.
However, it is not large enough to hold the entire flight
to which the certificate belongs, and hence there will
be fragmentation as soon as datagram packing is used.

This commit increases the maximum fragment length resp.
MTU values to 4096 bytes to ensure that even with datagram
packing in place, no fragmentation is necessary.

A similar change was made in "DTLS fragmenting: client (MTU)".

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