better code coverage

With #63 and #71 we have a much better framework for testing the full sizewatcher cli. Should add more tests, both e2e but also smaller unit tests (e.g. for `compare.js`).

Review issues #63 and #71 to understand the testing framework for the full sizewatcher CLI. Start with compare.js for unit-test opportunities, then identify the CLI end-to-end entry point; done means broader unit and end-to-end coverage, though this issue does not define specific cases or targets.

Written by the indexing model from the issue text.