Merge bitcoin/bitcoin#29898: test: Fix intermittent issue in p2p_handshake.py

6b02c11 test: Fix intermittent issue in p2p_handshake.py (stratospher)

Pull request description:

  When establishing outbound connections [`TestNode` --------> `P2PConnection`], `P2PConnection` listens for a single connection from `TestNode` on a [port which is fixed based on `p2p_idx`](https://github.com/bitcoin/bitcoin/blob/312f54278fd972ba3557c6a5b805fd244a063959/test/functional/test_framework/p2p.py#L746).

  If we reuse the same port when disconnecting and establishing connections again, we might hit this scenario where:
  - disconnection is done on python side for `P2PConnection`
  - disconnection not complete on c++ side for `TestNode`
  - we're trying to establish a new connection on same port again

  Prevent this scenario from happening by ensuring disconnection on c++ side for TestNode as well.

  One way to reproduce this on master would be adding a sleep statement before disconnection happens on c++ side.

  ```diff
  diff --git a/src/net.cpp b/src/net.cpp
  index e388f05..62507d1f39 100644
  --- a/src/net.cpp
  +++ b/src/net.cpp
  @@ -2112,6 +2112,7 @@ void CConnman::SocketHandlerConnected(const std::vector<CNode*>& nodes,
                   if (!pnode->fDisconnect) {
                       LogPrint(BCLog::NET, "socket closed for peer=%d\n", pnode->GetId());
                   }
  +                std::this_thread::sleep_for(std::chrono::milliseconds(1000));
                   pnode->CloseSocketDisconnect();
               }
               else if (nBytes < 0)
  ```

ACKs for top commit:
  maflcko:
    lgtm ACK 6b02c11
  mzumsande:
    Tested ACK 6b02c11
  BrandonOdiwuor:
    Tested ACK 6b02c11
  theStack:
    Tested ACK 6b02c11
  glozow:
    ACK 6b02c11

Tree-SHA512: 69509edb61ba45739fd585b6cc8a254f412975c124a5b5a52688288ecaaffd264dd76019b8290cc34c26c3ac2dfe477965ee5a11d7aabdd8e4d2a75229a4a068

Author: glozow

test/functional/p2p_handshake.py

@@ -41,6 +41,7 @@ def add_outbound_connection(self, node, connection_type, services, wait_for_disc
             peer.sync_with_ping()
             peer.peer_disconnect()
             peer.wait_for_disconnect()
+        self.wait_until(lambda: len(node.getpeerinfo()) == 0)
 
     def test_desirable_service_flags(self, node, service_flag_tests, desirable_service_flags, expect_disconnect):
         """Check that connecting to a peer either fails or succeeds depending on its offered