[test] unit test for new MockForward scheduler method

Summary:
This is a partial backport of Core 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