tests: enable clippy::get_unwrap lint

[laniakea64]

Going through our test runner code, I noticed I had written code that follows this pattern -

let h = HashMap::<String, String>::new();
let _ = h.get("foo").unwrap();

But the second line can use &h["foo"] for equivalent behavior, clearer code, and a more informative error message if that key unexpectedly doesn't exist. I've already changed our code to use this clearer style, but I was like, wait why didn't Clippy warn about this?

Turns out the answer is, there is such Clippy lint, but it's allow-by-default.

This PR makes .get(...).unwrap() a Clippy warning, i.e. it's fine in dev (e.g. temporarily using .unwrap() where error handling will be added later) but we don't want code like this in production.

[NoahTheDuke]

we don't want code like this in production.

lol