Chore: Refactor CLIEngine tests (refs #13481)

[nzakas]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request? (put an "X" next to an item)

[ ] Documentation update
[ ] Bug fix (template)
[ ] New rule (template)
[ ] Changes an existing rule (template)
[ ] Add autofixing to a rule
[ ] Add a CLI option
[ ] Add something to the core
[x] Other, please explain:

Refactored a test

What changes did you make? (Give an overview)

I removed the in-memory file system from the CLIEngine tests and replaced it with an actual filesystem. This is important for working on simple config because the in-memory filesystem does all kinds of complicated things to "fake" a filesystem, and it's impossible to reuse that to test that simple config doesn't break anything.

This change keeps all of the tests working the exact same way they did previously, only with an actual filesystem. That will allow me to eventually start using exports from @eslint/eslintrc inside CLIEngine and tell if I broke anything.

I'll need to do the same for the rest of the places in the tests where in-memory filesystems are used, so this is the first of a few such refactors.

Is there anything you'd like reviewers to focus on?

Anything obvious look out of place?

[mdjermanovic]

Looks good, I left some questions.

[nzakas]

Feedback addressed.

[btmills]

I saw two inconsistencies in your beforeEach/afterEach patterns. Not sure if that falls under the category of looking out of place, so I'll leave it to you to decide how much you care that they all follow the same pattern.

I left a code suggestion for a .skip that will demonstrate the broken ../.eslintrc.json not being cleaned up after its test. Hopefully that's enough to help you diagnose. Once that's resolved, LGTM.

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Helpful tip for future reviewers: updating the your comparison view settings to ignore whitespace will shave a few hundred lines off the diff.

[nzakas]

Updated to:

1. Explicitly delete the ../.eslintrc.json file in the one block where that's used.
2. Normalize the beforeEach/afterEach patterns. Note that in cases where cleanup is being overwritten inside a test case, I needed to use afterEach(() => cleanup()) instead of afterEach(cleanup) because the latter evaluates cleanup immediately (resulting in undefined) and the former evaluates cleanup at the time it executes.

[btmills]

Works for me! LGTM

[mdjermanovic]

Just two small notes. LGTM if you think there's no need to fix those at the moment.

[mdjermanovic]

LGTM, thanks!