tests: fixes mininode's P2PConnection sending messages on closing transport
tests: fixes mininode's P2PConnection sending messages on closing transport

- checks if  _transport.is_closing() (added in python3.4.4/python3.5.1)
before attempting to send messages on P2PConnection's send_message
method.

PIVX-Project/PIVX,PIVX-Project/PIVX,Darknet-Crypto/Darknet,PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,Darknet-Crypto/Darknet,Darknet-Crypto/Darknet,Darknet-Crypto/Darknet,PIVX-Project/PIVX,Darknet-Crypto/Darknet,PIVX-Project/PIVX,PIVX-Project/PIVX,Darknet-Crypto/Darknet