Figure out how to enable pyright's `reportUnnecessaryTypeIgnoreComment` setting for the whole `test_cases` directory

[AlexWaygood]

Currently, we have mypy's --warn-unused-ignore setting enabled for the whole test_cases directory, but for pyright, we have to manually add # pyright: reportUnnecessaryTypeIgnoreComment=true at the top of each file in the directory. These settings are crucial to these tests working properly; none of our tests against false-negatives work properly without them.

It would be good if we could work out how to get # pyright: reportUnnecessaryTypeIgnoreComment=true enabled for the whole directory. The current system is error-prone: if we forget to add it at the top of a file, any tests against false negatives could fail silently on pyright.