refactor(tests): split up test suites, update docs

[vados-cosmonic]

No description provided.

[vados-cosmonic]

OK, so this PR is ready for review, but I'm a little dissatisfied with the consistency of CI -- running with more concurrency seems to have made tests more unstable.

Going to re-run the latest job a few more times to make sure it passes most of the time so that the average PR should pass without issues.

[vados-cosmonic]

OK unfortunately there were still a couple failures -- (mac/windows + weval). I wonder if we can get away with disabling test concurrency for weval runs... If that's more consistent then I'll go with that, otherwise I'll just disable test-level concurrency all together (suite level concurrency still happens)

[vados-cosmonic]

OK, going to rip out concurrency here -- tests consistently passing is more important I think.

[EDIT] - Actually the test that is still failing is a Weval one which is timing out... Updating test config to run only one concurrent suite at a time if we're doing that.

[tschneidereit]

@vados-cosmonic same request I often have: can you update the description here to add some motivation for this? It's so very much easier to review PRs when I know which lens to look through. It's also very useful for looking at changes later on.

[vados-cosmonic]

Hey @tschneidereit yeah it's been a while on this PR -- I believe I was trying to finish the switch to vitest in this PR, in particular a follow up to this PR:

#277

In particular this comment here:

#277 (comment)

When not in CI (on really slow machines), this PR makes the tests finish faster by separating one large suite into multiple (that can run in parallel under vitest).