[tests] Make NodeConnCB a subclass of NodeConn

This makes NodeConnCB a subclass of NodeConn, and
removes the need for the client code to know
anything about the implementation details of NodeConnCB.

NodeConn can now be swapped out for any other implementation
of a low-level connection without changing client code.

test/functional/assumevalid.py

@@ -67,7 +67,7 @@ class AssumeValidTest(BitcoinTestFramework):
     def send_blocks_until_disconnected(self, p2p_conn):
         """Keep sending blocks to the node until we're disconnected."""
         for i in range(len(self.blocks)):
-            if not p2p_conn.connection:
+            if p2p_conn.state != "connected":
                 break
             try:
                 p2p_conn.send_message(msg_block(self.blocks[i]))