Force IPv6 for DTLS interop tests with gnutls-cli
Force IPv6 for DTLS interop tests with gnutls-cli

Since `gnutls-cli` resolves `localhost` as an IPv6 address, and the server
is bound to IPv4 address, gnutl-cli fails to negotiate DTLS sessions.
Force the server to bind to IPv6 address, as a workaround.

Signed-off-by: Johan Pascal <a9ed27abf9fd2327f3cdfbacd27d3e6c0500f5a6@belledonne-communications.com>

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