[test] unit test for new MockForward scheduler method
[test] unit test for new MockForward scheduler method

Summary:
This is a partial backport of Core [[https://github.com/bitcoin/bitcoin/pull/18037 | PR18037]] : https://github.com/bitcoin/bitcoin/pull/18037/commits/1cd43e83c6e8d81e950aaaede7a8a51505d0a2bc

Depends on D6499

Test Plan:
  ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6500

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