We test rules with a custom rule tester doing a snapshot test.

The built-in RuleTester can't customize the output assertion, we don't have the output to send to RuleTester, so we have to use SourceCodeFixer to apply fix from messages. Full implementation

We definitely want a better way to do snapshot test, maybe pass an assert option in future, but before that, can we expose SourceCodeFixer, so I can apply fix from the message?

The version of ESLint you are using.

8.0.0-beta.0

The problem you want to solve.

Fix our rule tester.

Are you willing to submit a pull request to implement this change?

Yes.