feat: Hooks for test cases

[RobinTail]

Summary

I propose adding an optional setup before (and after) properties to the test cases that the ESLint RuleTester runs. This
feature will allow developers to prepare specific environments needed for certain rules before running each test case.

Related Issues

• Prior issue: RuleTester feature: setup for test cases eslint#18770
• Draft implementation PR: feat: setup option for a test case (RuleTester) eslint#18771

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[jfmengels]

Do you think setup will in practice only be used to set up test files? I can imagine it also being used for environment variables (though I don't know if any rules do that in practice), but I'm failing to see other use-cases.

Some prior art on the subject: elm-review allows looking at multiple files, including the elm.json file (the equivalent of package.json, example). Because this is supported out of the box - and rules have no means to do side-effects to load files like is done by no-extraneous-dependencies (my baby from 8 years ago, time flies) - there is a way in tests to construct a mock project (example) and to use that in a test (example).

I think that if ESLint ever adds native support for reading package.json files or for multi-file analysis, then an test API dedicated for setting the value of files will be preferable.

In the meantime, I guess that a setup function property does the trick, but as mentioned, it can be misused and can lead to interdependent tests.

[RobinTail]

Do you think setup will in practice only be used to set up test files?

I don't.

[jfmengels]

@RobinTail What else do you think it will or could be used for in practice?

[RobinTail]

I know what I need it for, but humans in general are very inventive beings, @jfmengels , some of them find very unusual applications for quite ordinary items.

[RobinTail]

• setup renamed to before
• added after
• mentioned the duplicates detection drawback

[mdjermanovic]

Overall LGTM, just few more details to clarify.

[nzakas]

@RobinTail there is some other feedback to address. Please take a look.

[RobinTail]

yeah, sorry for delay — something came up and I'll get back to this soon.
thanks for reminder, @nzakas

[RobinTail]

Addressed, @nzakas

[mdjermanovic]

Looks good to me, thanks!

[nzakas]

LGTM. Moving to final commenting.

[fasttime]

LGTM, thanks.