🧪 Tests: Add end-to-end tests?

[JoshuaKGoldberg]

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Overview

There've been a bunch of (excellent) issues filed by @rubiesonthesky (e.g. #1498) as a result of running TypeStat on this repo. Which indicates there's not enough test coverage to exercise TypeStat on larger repos such as itself.

Proposal: how about adding some end-to-end tests that truly run TypeStat -not just the existing "mutation" (integration-style) tests- on full repositories?

• TypeStat itself?
• A few small example repos of various styles (JS, mixed-JS-TS, TS)
• Perhaps as a followup: cloning known good repos at particular commits and running on them?

This would come after #1476.