Lots of work here, mostly because I didn't want to turn off the
`noImplicitAnyLet` lint. This PR tries its best to type all the untyped
lets biome complained about (Don't ask me how many hours that took or
how many lints that was >200...), which in the future will force test
authors to actually type their global variables setup in `beforeAll`.
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Co-authored-by: Gastón Fournier <gaston@getunleash.io>
