Add txrequest unit tests

Summary:

Add unit tests for TxRequestTracker. Several scenarios are tested,
randomly interleaved with eachother.

Includes a test by Antoine Riard (ariard).

Partial backport of core#19988:
https://github.com/bitcoin/bitcoin/pull/19988/commits/3c7fe0e5a0ee1abf4dc263ae5310e68253c866e1

Partial backport of core#20162.

Depends on D9547.

Test Plan:

ninja check

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9548