Add small packet tests for DTLS
Add small packet tests for DTLS

Add a DTLS small packet test for each of the following combinations:
- DTLS version: 1.0 or 1.2
- Encrypt then MAC extension enabled
- Truncated HMAC extension enabled

Large packets tests for DTLS are currently not possible due to parameter
constraints in ssl_server2.

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