Add a TLS message (e.g. TLSClientHello or TLSApplicationData)
        inside the latest record to be sent through the socket.
        We believe a good automaton should not use the first test.

def add_msg(self, pkt):
        """
        Add a TLS message (e.g. TLSClientHello or TLSApplicationData)
        inside the latest record to be sent through the socket.
        We believe a good automaton should not use the first test.
        """
        if not self.buffer_out:
            self.add_record()
        r = self.buffer_out[-1]
        if isinstance(r, TLS13):
            self.buffer_out[-1].inner.msg.append(pkt)
        else:
            self.buffer_out[-1].msg.append(pkt)