Feature Request Checklist

• I have read and agree to Mocha's Code of Conduct and Contributing Guidelines
• I have searched for related issues and issues with the faq label, but none matched my issue.
• I want to provide a PR to resolve this

Overview

It's an unfortunately common error to check in a test file with .only. This can hide failures especially when run in a CI system where the number of tests is not reported or easily viewable.

Suggested Solution

Automatically fail the test run when a .only is found and emit exit code 1.

(This is the behavior of newer test runners such as the builtin Node.js and Deno runners.)

Alternatives

Using a different testing framework. 😄

Additional Info

No response