Merge bitcoin/bitcoin#22249: test: kill process group to avoid dangling processes when using `--failfast`

451b96f7d2796d00eabaec56d831f9e9b1a569cc test: kill process group to avoid dangling processes (S3RK)

Pull request description:

  This is an alternative to #19281

  This PR fixes a problem when after test failure with `--failfast` option there could be dangling nodes. The nodes will continue to occupy rpc/p2p ports on the machine and will cause further test failures.

  If there are any dangling nodes left at the end of the test run we kill the whole process group.
  Pros: the operations is immediate and won't lead to CI timeout
  Cons: the test_runner process is also killed and exit code is 137

  Example output:
  ```
  ...
  Early exiting after test failure

  TEST                           | STATUS    | DURATION

  rpc_decodescript.py            | ✓ Passed  | 2 s
  rpc_deprecated.py              | ✓ Passed  | 2 s
  rpc_deriveaddresses.py         | ✓ Passed  | 2 s
  rpc_dumptxoutset.py            | ✖ Failed  | 2 s

  ALL                            | ✖ Failed  | 8 s (accumulated)
  Runtime: 4 s

  Killed: 9
  > echo $?
  137
  ```

ACKs for top commit:
  MarcoFalke:
    review ACK 451b96f7d2796d00eabaec56d831f9e9b1a569cc
  aitorjs:
    ACK 451b96f7d2796d00eabaec56d831f9e9b1a569cc. Manual testing with and without **--failfast**.

Tree-SHA512: 87e510a1411b9e7571e63cf7ffc8b9a8935daf9112ffc0f069d6c406ba87743ec439808181f7e13cb97bb200fad528589786c47f0b43cf3a2ef0d06a23cb86dd

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