ssl-opt.sh: add proxy to all DTLS tests
ssl-opt.sh: add proxy to all DTLS tests

A lot of DTLS test are timing-sensitive, especially those that contain
assertions about retransmission. Sometimes some DTLS test fails intermittently
on the CI with no clear apparent reason; we need more information in the log
to understand the cause of those failures.

Adding a proxy means we'll get timing information from the proxy logs.

An alternative would be to add timing information to the debug output of
ssl_server2 and ssl_client2. But that's more complex because getting
sub-second timing info is outside the scope of the C standard, and our current
timing module only provides a APi for sub-second intervals, not absolute time.
Using the proxy is easier as it's a single point that sees all messages, so
elapsed time is fine here, and it's already implemented in the proxy output.

Signed-off-by: Manuel Pégourié-Gonnard <4ba0d615aeeda88e422ca8fba28fe2938485facf@arm.com>

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