Merge #13715: tests: fixes mininode's P2PConnection sending messages on closing transport

ea5340c tests: fixes mininode's P2PConnection sending messages on closing transport (marcoagner)

Pull request description:

  Fixes #13579.
  I think one possible solution is to check for [`_transport.is_closing()`](https://docs.python.org/3.4/library/asyncio-protocol.html#asyncio.BaseTransport.is_closing) in the lambda before sending a message (compatible with Python 3.4 too). Let me know if I missed any side effects this introduces.

Tree-SHA512: cab46f81dccfec7b4460fda478a617845564520694449a9e85bf8a5f1e75f35f52cafd7c64966712c3d6c29956344d5a9dbad8851424f061eb3748bc621b900b

Author: MarcoFalke

test/functional/test_framework/mininode.py

@@ -179,7 +179,7 @@ def send_message(self, message):
             raise IOError('Not connected')
         self._log_message("send", message)
         tmsg = self._build_message(message)
-        NetworkThread.network_event_loop.call_soon_threadsafe(lambda: self._transport and self._transport.write(tmsg))
+        NetworkThread.network_event_loop.call_soon_threadsafe(lambda: self._transport and not self._transport.is_closing() and self._transport.write(tmsg))
 
     # Class utility methods