test: Fix feature_config_args.py intermittent issue

Summary:
See failure in D16248.

Backport of core#21814.

Test Plan:

./test/functional/test_runner.py feature_config_args

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16251