Message to indicate the sending of a special response in unit tests

tests/unit-test-server.c

@@ -130,6 +130,7 @@ int main(int argc, char*argv[])
                                          UT_REGISTERS_NB_POINTS);
             } else if (MODBUS_GET_INT16_FROM_INT8(query, header_length + 1)
                 == UT_REGISTERS_ADDRESS_SPECIAL) {
+                printf("Reply to this special register address by an exception\n");
                 modbus_reply_exception(ctx, query,
                                        MODBUS_EXCEPTION_SLAVE_OR_SERVER_BUSY);
                 continue;