Add unit tests for TxRequestTracker. Several scenarios are tested,
randomly interleaved with eachother.
Includes a test by Antoine Riard (ariard).
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Partial backport of [[https://github.com/bitcoin/bitcoin/pull/19988 | core#19988]]
https://github.com/bitcoin/bitcoin/pull/19988/commits/3c7fe0e5a0ee1abf4dc263ae5310e68253c866e1
Depends on D9536