[test] P2P connection behavior should meet expectations

- P2PTxInvStore should supplement `on_inv` behavior of parent, not overwrite.

Author: amitiuttarwar

test/functional/test_framework/mininode.py

@@ -652,6 +652,8 @@ def on_inv(self, message):
                 # save txid
                 self.tx_invs_received[i.hash] += 1
 
+        super().on_inv(message)
+
     def get_invs(self):
         with mininode_lock:
             return list(self.tx_invs_received.keys())