Bug: Unable to test rules in v9 in both flat config and eslintrc about eslint

[aladdin-add]

I'm 👍 to the change.

Remember: users can still use eslintrc with eslint v9 (ESLINT_USE_FLAT_CONFIG=false), but it's no longer testable.

[nzakas]

As we announced, RuleTester in v9 now tests against flat config because that is the default configuration system.

Functionally, if your rules pass RuleTester in v9, they should work with eslintrc also, as the fundamental parts of rule tester remain the same, it's really just the format of the options that have changed.

If you really need to test eslintrc specifically, then you'll need to use ESLint v8.x for that.

[ljharb]

It makes perfect sense for RuleTester to test it by default, but I don't understand why eslint 9, which still supports two config systems, has failed to provide a way to test both.