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 <manuel.pegourie-gonnard@arm.com>

tests/ssl-opt.sh

@@ -675,6 +675,12 @@ run_test() {
     # update DTLS variable
     detect_dtls "$SRV_CMD"
 
+    # if the test uses DTLS but no custom proxy, add a simple proxy
+    # as it provides timing info that's useful to debug failures
+    if [ "X$PXY_CMD" = "X" -a "$DTLS" -eq 1 ]; then
+        PXY_CMD="$P_PXY"
+    fi
+
     # prepend valgrind to our commands if active
     if [ "$MEMCHECK" -gt 0 ]; then
         if is_polar "$SRV_CMD"; then