wallet refactor: Avoid use of Chain::Lock in qt wallettests

Summary:
This is a step toward removing the Chain::Lock class and reducing cs_main
locking.

This change doesn't affect behavior.

This is part [3/12] of Core PR17954 : https://github.com/bitcoin/bitcoin/pull/17954/commits/ade5f87971211bc67753f14a0d49e020142efc7c

Depends on D7865

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7866