chore: put `__tests__/.eslintrc.json` in root's config's `override`

[nschonni]

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What is the purpose of this pull request?

• [ ] Documentation update
• [ ] Bug fix
• [ ] New rule
• [ ] Changes an existing rule
• [ ] Add autofixing to a rule
• [x] Other, please explain:

What changes did you make? (Give an overview) Merged up the nested ESLint config to an override in the root one

[xjamundx]

Tell me more about why this is better. The old way made sense to me. This also seems fine. Just wondering if there's a big benefit.

[nschonni]

Yeah, either way is fine. Using the single file, you get to see all the rules and overrides in a single location when you want to add/remove plug-ins

[voxpelli]

Something like this will need to be done when we adapt to flat config anyway, but I think it's better to focus on that rather than do this now that flat configs are the future

[brettz9]

And FWIW, FWICT, with flat configs at least, the "overrides" equivalent does not wipe out the whole property.