Run Pyright on test files to catch typing issues

[benhoyt]

We currently don't run Pyright over our test/*.py files to type-check those, meaning we're not getting "coverage" of most of our type annotations, which results in issues like #963.

We should enable Pyright on these files. I suspect this will be a fairly big diff -- if so, we may want to opt the files in one by one in separate PRs to streamline review.