qa: Fix double-negative arg test

Summary:
The test is made more robust by not accept trailing characters after 0.
This change was made after the test failed to fail when a bug was introduced (pointer address used instead of the string in a LogPrintf call).

This is a backport of Core PR17893

Test Plan: ninja && test/functional/test_runner.py feature_config_args.py

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8424