qa: Increase includeconf test coverage

Summary:
Backport of core PR13367:
https://github.com/bitcoin/bitcoin/pull/13367/files

Test Plan:

make check
test_runner.py feature_includeconf feature_config_args

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4509