test: update log message and use SYNTHETIC_ERRNO()

Follow-up for ce5a6d5307.

Addresses https://github.com/systemd/systemd/pull/30049#discussion_r1395453408.

src/libsystemd/sd-bus/test-bus-chat.c

@@ -432,8 +432,7 @@ static void* client2(void *p) {
         if (r < 0)
                 log_debug("Failed to issue method call: %s", bus_error_message(&error, r));
         else {
-                log_error("Slow call unexpectedly succeed.");
-                r = -ENOANO;
+                r = log_error_errno(SYNTHETIC_ERRNO(ENOANO), "Slow call unexpectedly succeeded.");
                 goto finish;
         }