docs: add stricter rule test validations to v9 migration guide (#18085)

docs/src/use/migrate-to-9.0.0.md

@@ -543,9 +543,16 @@ As announced in our [blog post](/blog/2023/10/flat-config-rollout-plans/), the t
 
 In order to aid in the development of high-quality custom rules that are free from common bugs, ESLint v9.0.0 implements several changes to `RuleTester`:
 
+1. **Test case `output` must be different from `code`.** In ESLint v8.x, if  `output` is the same as `code`, it asserts that there was no autofix. When looking at a test case, it's not always immediately clear whether `output` differs from `code`, especially if the strings are longer or multiline, making it difficult for developers to determine whether or not the test case expects an autofix. In ESLint v9.0.0, to avoid this ambiguity, `RuleTester` now throws an error if the test `output` has the same value as the test `code`. Therefore, specifying `output` now necessarily means that the test case expects an autofix and asserts its result. If the test case doesn't expect an autofix, omit the `output` property or set it to `null`. This asserts that there was no autofix.
+1. **Test error objects must specify `message` or `messageId`.** To improve the quality of test coverage, `RuleTester` now throws an error if neither `message` nor `messageId` is specified on test error objects.
+1. **Test error object must specify `suggestions` if the actual error provides suggestions.** In ESLint v8.x, if the `suggestions` property was omitted from test error objects, `RuleTester` wasn't performing any checks related to suggestions, so it was easy to forget to assert if a test case produces suggestions. In ESLint v9.0.0, omitting the `suggestions` property asserts that the actual error does not provide suggestions, while you need to specify the `suggestions` property if the actual error does provide suggestions. We highly recommend that you test suggestions in detail by specifying an array of test suggestion objects, but you can also specify `suggestions: <number>` to assert just the number of suggestions.
+1. **Test suggestion objects must specify `output`.** To improve the quality of test coverage, `RuleTester` now throws an error if `output` property is not specified on test suggestion objects.
+1. **Test suggestion objects must specify `desc` or `messageId`.** To improve the quality of test coverage, `RuleTester` now throws an error if neither `desc` nor `messageId` property is specified on test suggestion objects. It's also not allowed to specify both. If you want to assert the suggestion description text in addition to the `messageId`, then also add the `data` property.
 1. **Suggestion messages must be unique.** Because suggestions are typically displayed in an editor as a dropdown list, it's important that no two suggestions for the same lint problem have the same message. Otherwise, it's impossible to know what any given suggestion will do. This additional check runs automatically.
+1. **Suggestions must change the code.** Suggestions are expected to fix the reported problem by changing the code. `RuleTester` now throws an error if the suggestion test `output` is the same as the test `code`.
 1. **Suggestions must generate valid syntax.** In order for rule suggestions to be helpful, they need to be valid syntax. `RuleTester` now parses the output of suggestions using the same language options as the `code` value and throws an error if parsing fails.
 1. **Test cases must be unique.** Identical test cases can cause confusion and be hard to detect manually in a long test file. Duplicates are now automatically detected and can be safely removed.
+1. **`filename` and `only` must be of the expected type.** `RuleTester` now checks the type of `filename` and `only` properties of test objects. If specified, `filename` must be a string value. If specified, `only` must be a boolean value.
 
 **To address:** Run your rule tests using `RuleTester` and fix any errors that occur. The changes you'll need to make to satisfy `RuleTester` are compatible with ESLint v8.x.