RFC: https://github.com/eslint/rfcs/tree/main/designs/2024-hooks-for-test-cases

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 autofix to a rule
[ ] Add a CLI option
[X] Add something to the core
[ ] Other, please explain:

Closes #18770 — Developers can mock the environment right within the test cases instead of maintaining fixture files on disk.

What changes did you make? (Give an overview)

• added setup property to the test case (both valid and invalid);
  • the prop is optional and the change is backwards compatible;
• if the setup is present:
  • when assigned with a function, then it calls that function before testing the case;
  • otherwise throws;
• added tests for both scenarios;
• listed the new property in the documentation;

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