New: Add only to RuleTester (refs eslint/rfcs#73) #14677
What is the purpose of this pull request? (put an "X" next to an item)
[ ] Documentation update
What changes did you make? (Give an overview)
This implements RFC73.
Is there anything you'd like reviewers to focus on?
Makes sense to me, some customizations might "just work". If it doesn't work by itself, they can override