test: fix intermittent failure in p2p_sendtxrcncl.py

Using disconnect_p2ps instead of peer_disconnect makes
the node wait for the disconnect to complete. As a result,
we can reuse p2p_idx=0 in the add_outbound_p2p_connection calls.

test/functional/test_framework/test_node.py

@@ -625,6 +625,10 @@ class TestNode():
 
         This method adds the p2p connection to the self.p2ps list and returns
         the connection to the caller.
+
+        p2p_idx must be different for simultaneously connected peers. When reusing it for the next peer
+        after disconnecting the previous one, it is necessary to wait for the disconnect to finish to avoid
+        a race condition.
         """
 
         def addconnection_callback(address, port):