- test passing tsconfig, buggy tsconfig, non-existent tsconfig, and not a tsconfig
- clean: "failed to read" error will never happen as we already checked for existence of the file earlier
  - so remove the `undefined` check and instead use a non-null assertion (plus a comment explaining it)

- refactor: move the integration test for tsconfig error into this unit test instead
  - faster / more efficient / more precise

- refactor: split out a `makeOptions` func that creates default plugin options to use in tests
  - similar to `makeStubbedContext`