Merge #11182: [tests] Add P2P interface to TestNode

Merge #11182: [tests] Add P2P interface to TestNode

32ae82f5c [tests] use TestNode p2p connection in tests (John Newbery)
5e5725cc2 [tests] Add p2p connection to TestNode (John Newbery)
b86c1cd20 [tests] fix TestNode.__getattr__() method (John Newbery)

Pull request description:

  Final two steps of #10082 : Adding the "mininode" P2P interface to `TestNode`

  This PR adds the mininode P2P interface to `TestNode`. It simplifies the process for opening a P2P connection to the node-under-test from this:

  ```python
  node0 = NodeConnCB()
  connections = []
  connections.append(NodeConn('127.0.0.1', p2p_port(0), self.nodes[0], node0))
  node0.add_connection(connections[0])
  ```

  to this:

  ```python
  self.nodes[0].add_p2p_connection(p2p_conn_type=NodeConnCB)
  ```

  The first commit adds the infrastructure to `test_node.py`. The second updates the individual test cases to use it. Can be separated if this is too much review for one PR.

Tree-SHA512: 44f1a6320f44eefc70489ae8350c6a16ad1a6035e4b9b7bafbdf19f5905ed0e2db85beaaf4758eec3059dd89a375a47a45352a029f39f57a86ab38a9ae66650e