test/util: Use existing chainman in ::PrepareBlock

Summary:
This is a backport of core#21866 [2/12]
https://github.com/bitcoin/bitcoin/pull/21866/commits/f0dd5e6bb4b16e69d35b648b7ef973a732229873

Depends on D11646

Test Plan: ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11647