test/setup: Use LoadChainstate

Summary:

This commit coalesces the chainstate loading sequence between our unit test and non-unit test init codepaths.

Partial backport of core#23280:
https://github.com/bitcoin/bitcoin/pull/23280/commits/9a5a5a3d08b2c130ab9147914739ff3583b0dc84

Depends on D12582.

Test Plan:

ninja all check

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D12583