Merge #18890: test: disconnect_nodes should warn if nodes were already disconnected

34e641a564531853342b03db2d9f0bf52b6e439e test: Remove unnecessary disconnect_nodes call in rpc_psbt.py (Danny Lee)
e6e7abd51a9a6027acac7a9964e36357f25e242c test: remove redundant two-way disconnect_nodes calls (Danny Lee)
a9bd1f9adf869a95f70b3a40615a2f8e8e52db1d test: warn if nodes not connected before disconnect_nodes (Danny Lee)

Pull request description:

  There's no harm in calling `disconnect_nodes` for nodes that weren't connected (in this case it's a no-op).  However, detecting this case and logging a warning can help ensure that tests are behaving as expected.

  In addition, since `disconnect_nodes` works bidirectionally, I removed all instances of this pattern:

  ```
  disconnect_nodes(self.nodes[0], 1)
  disconnect_nodes(self.nodes[1], 0)
  ```

ACKs for top commit:
  MarcoFalke:
    review ACK 34e641a564531853342b03db2d9f0bf52b6e439e 👔
  amitiuttarwar:
    ACK 34e641a564. Thanks for this test improvement!

Tree-SHA512: 344855ceb46c012d43c13d7c09f44d32dcb7645706d10ae1e4645d9edca54c6c6c13fee26b79480755cdfcdf39b4b5770b36bb03ce71ba002d5be8a27fe008af

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