Merge #19804: test/refactor: reference p2p objects explicitly and remove confusing Test_Node.p2p property

10d61505fe77880d6989115defa5e08417f3de2d [test] remove confusing p2p property (gzhao408)
549d30faf04612d9589c81edf9770c99e3221885 scripted-diff: replace p2p with p2ps[0] in p2p_invalid_tx (gzhao408)
7a0de46aeafb351cffa3410e1aae9809fd4698ad [doc] sample code for test framework p2p objects (gzhao408)
784f757994c1306bb6584b14c0c78617d6248432 [refactor] clarify tests by referencing p2p objects directly (gzhao408)

Pull request description:

  The `TestNode` has a `p2p` property which is an alias for `p2ps[0]`.

  I think this should be removed because it can be confusing and misleading (to both the test writer and reviewer), especially if a TestNode has multiple p2ps connected (which is the case for many tests).
  Another example is when a test has multiple subtests that connect 1 p2p and use the `p2p` property to reference it. If the subtests don't completely clean up after themselves, the subtests may affect one another.

  The best way to refer to a connected p2p is use the object returned by `add_p2p_connection` like this:
  ```py
  p2p_conn = node.add_p2p_connection(P2PInterface())
  ```
  A good example is [p2p_invalid_locator.py](https://github.com/bitcoin/bitcoin/blob/master/test/functional/p2p_invalid_locator.py), which cleans up after itself (waits in both `wait_for_disconnect` and in `disconnect_p2ps`) but wouldn't need so much complexity if it just referenced the connections directly.

  If there is only one connected, it's not really that tedious to just use `node.p2ps[0]` instead of `node.p2p` (and it can always be aliased inside the test itself).

ACKs for top commit:
  robot-dreams:
    utACK 10d61505fe77880d6989115defa5e08417f3de2d
  jnewbery:
    utACK 10d61505fe77880d6989115defa5e08417f3de2d
  guggero:
    Concept ACK 10d61505.

Tree-SHA512: 5965548929794ec660dae03467640cb2156d7d826cefd26d3a126472cbc2494b855c1d26bbb7b412281fbdc92b9798b9765a85c27bc1a97f7798f27f64db6f13

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