[posix] set message origin to host trusted for reference devices

Reference devices that are part of the Thread test harness construct CoAP packets sent on TMF port 61631 for certain tests, for example in the 5.9.x series where they have to force address errors (a/ae) for duplicate DUA or re-registration tests. These tests started to fail when reference device firmware was updated recently to a newer OpenThread stack that included the change in openthread#9437.

Example:
```
udp send fd00:db9:0:0:0:ff:fe00:5000 61631 -x 4102d63697b16e02646eff0401010010fd007d037d037d0389c3a350cdcf36e0'
Ip6-----------: Dropping TMF message from untrusted origin
```

For certification purposes, we need to trust messages sent by reference devices.

src/posix/platform/netif.cpp

@@ -1232,7 +1232,11 @@ static void processTransmit(otInstance *aInstance)
         message = otIp6NewMessage(aInstance, &settings);
 #endif
         VerifyOrExit(message != nullptr, error = OT_ERROR_NO_BUFS);
+#if OPENTHREAD_CONFIG_REFERENCE_DEVICE_ENABLE
+        otMessageSetOrigin(message, OT_MESSAGE_ORIGIN_HOST_TRUSTED);
+#else
         otMessageSetOrigin(message, OT_MESSAGE_ORIGIN_HOST_UNTRUSTED);
+#endif
     }
 
 #if OPENTHREAD_POSIX_LOG_TUN_PACKETS