Missing: Mechanism for unit tests which test cross-file situations

https://github.com/captbaritone/grats/commit/1ccab75259c0de65a3436ed6adf496c05a50e39b adds a change how we handle multiple files each with an error, but we have no test coverage for this, nor any ability to validate the behavior.

Start by reviewing commit 1ccab75259c0de65a3436ed6adf496c05a50e39b and the repository's existing unit-test setup. Identify how tests currently represent multiple files, then add coverage for the cross-file error behavior and ensure the test mechanism can validate it.

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