[util] allow scheduler to be mocked

Summary:
Add MockForward method to the scheduler that mimics going into the future by rescheduling all items on the taskQueue to be sooner.

This is a partial backport of Core PR18037 : https://github.com/bitcoin/bitcoin/pull/18037/commits/a6f63598adb880a75e1571aac58338c17fa7ad53

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6499