qt/test: Use existing chainman in ::TestGUI

Summary:
This is a backport of core#21866 [8/12]
https://github.com/bitcoin/bitcoin/pull/21866/commits/f323248aba5088c9630e5cdfe5ce980f21633fe8

Test Plan: ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11684