init: Setup scheduler in tests and init in exactly the same way

Summary:
This is a partial backport of core#19028
https://github.com/bitcoin/bitcoin/pull/19028/commits/fa4ea997b4da1ae0afafba223fff9efbeefaf555

This is backported as a minor dependency of core#21016

Test Plan:
With clang and debug:
ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10991