test: call VerifyLoadedChainstate during ChainTestingSetup

Summary:
for additional coverage and similarity to actual init process.

This is a backport of core#23736

Test Plan: ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D14584