add tests for validating `helper.py` CLI interactions

We recently had a regression where the reproducer command stopped providing the output from the container run, which meant that reproducer commands just stopped silently. We should improve such that we have tests that are a bit more rigid in terms of enforcing the CLI interaction we expect from `helper.py`.

Refs:
- regression: https://github.com/google/oss-fuzz/pull/14145
- fix 1: https://github.com/google/oss-fuzz/pull/14178
- fix 2: https://github.com/google/oss-fuzz/pull/14299

Locate helper.py and any existing tests for its CLI interactions, then review the referenced regression and fixes to understand the expected container-run output. Add tests that fail when that output is not provided and pass when the expected interaction is preserved.

Written by the indexing model from the issue text.