an internal TX block STOP stash path is unreachable from an external C++ TB

the internal TX block's stop_c*_p* flags are registered, so STOP injected after read_en has already fired lands one cycle late. SEND_DATA_FIFO checks the old value and sends without stashing into stp_cmd_f*. The stash path needs STOP to arrive during a throttle-backoff wait, which is hard to time from outside. Cover it with a dedicated unit test for the internal TX block instead of a system-level test.