Do not use LocalTestingSetup in getarg_tests test file.

Summary:
Avoid using a test fixture in getarg_tests for better readability.

This is a backport of core#24375

Depends on D15263

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15264