chore: stop using internal Relude_* in tests

[mlms13]

This is a chunk of the work that was happening in @trite's dunification branch. Specifically, this makes it so all of the tests in __tests__ no longer use Relude_* modules, which are internal and won't be available to the tests module once we properly set up dune packages.

[mlms13]

Ugh... never mind. I forgot that in the pre-dunification branch, everything requires esy to build, and I can't seem to npm install -g esy on my Mac without hitting weird code signing errors. Not sure I have the patience to tackle this independently of the dune work, so I'm just going to close this PR.