Cleanups some tests

[arcanis]

What's the problem this PR addresses?

I'm reviewing our tests and came upon some of them that could be dumbed down. I prefer to make them test as little things as possible to avoid something breaking due to unrelated parts of the code changing (I'm working on switching some implementations, and limiting test dependencies make things considerably easier / more fun to work on).

How did you fix it?

No big changes - it's mostly things like using the dependencies rather than calling yarn add and avoiding to rely on cache paths to check whether something got properly installed.

Checklist

• I have read the Contributing Guide.

• I have set the packages that need to be released for my changes to be effective.

• I will check that all automated PR checks pass before the PR gets reviewed.