Modify "util_datadir" unit test to not use gArgs.

Summary:
This is a backport of core#21244 [3/8]
https://github.com/bitcoin/bitcoin/pull/21244/commits/1cb52ba0656e78ca6c2ef84b1558198ad113b76a

Depends on D10750

Test Plan: ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10751