Wrote an additional test for TCPSender

[hunterZh37]

I previous wrongly implemented TCPSenderMessage TCPSender::make_empty_message() with the following implementation because I thought you had to RST when you send an empty message & this passed all the tests!

TCPSenderMessage TCPSender::make_empty_message() const
{
  TCPSenderMessage sender_message {};
  sender_message.seqno = Wrap32::wrap( next_abs_seqno_, isn_ );
  sender_message.RST = true; // WRONG! Should rather be sender_message.RST = reader().has_error();
  return sender_message;
}

After studying, I realized that the correct implementation should be: sender_message.RST = reader().has_error();

That means the current test case is always expecting RST to be true when make_empty_message() is called, but this should not be the case because RST should only be true when reader().has_error() is true.