Enhancement(rule-tester): assert suggestion messages are unique

[JoshuaKGoldberg]

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• I have searched for related issues and found none that match my proposal.
• I have searched the current rule list and found no rules that match my proposal.
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Relevant Package

rule-tester

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• I believe my proposal would be useful to the broader TypeScript community (meaning it is not a niche proposal).

Description

From #8211: there are quite a few enhancements to the ESLint RuleTester that we'll need to reflect in our own rule-tester class.

• Rule Tester checks for missing placeholder data in the message
• Stricter rule test validations
• detect duplicate test cases
• check for parsing errors in suggestion fixes
• assert suggestion messages are unique in rule testers

Per the discussion in that thread, we'd like to not treat this as a breaking change:

TBH I'm actually okay with us updating @typescript-eslint/rule-tester with these changes. Yes it will cause errors in tests for people - but they're all correct errors that should have been caught previously. I think it's okay treating it the same as a lint rule i.e. "A change to the plugins shall not be considered breaking if it: ... Adds additional checks to an existing rule that causes new reports in a small-to-medium set of cases in an average codebase".

...so unless someone protests, we'll merge this into the main branch rather than v8.

Additional Info

Adding to the v8 milestone as it's required for ESLint 9 support, even though we'd like to land this sooner.

💖

[JoshuaKGoldberg]

For the sake of easier tracking, retargeting this issue to be specifically about the first of the changes (assert suggestion messages are unique). Filing new issues for the rest of them.

#8995 was merged into main, so marking this as fixed by #8995.