🐛 Bug: this.fn is not defined in Runnable.prototype.run

[quinton-ashley]

Bug Report 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 have 'smoke tested' the code to be tested by running it outside the real test suite to get a better sense of whether the problem is in the code under test, my usage of Mocha, or Mocha itself.
• I want to provide a PR to resolve this

Expected

this.fn is not defined in Runnable.prototype.run when called internally on runner.js ln 509

Problem exists in v10.8.2

Actual

No error

Minimal, Reproducible Example

No

Versions

v10.8.2

Additional Info

No response