test: Change background_cs from pointer to reference in validation_chainstate_tests

Summary:
This changes background_cs from being a pointer to a reference to work
around a gcc false warning. Also, this makes the test easier to read.

Fixes https://github.com/bitcoin/bitcoin/issues/23101

Can be reviewed with --ignore-all-space.

This is a backport of core#23132

Test Plan: ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15006