Update CONTRIBUTING.md

[compulim]

Changelog Entry

(No changelog for docs)

Description

Update CONTRIBUTING.md after we updated our test harness in #3871.

Specific Changes

• Updated CONTRIBUTING.md
• Added a screenshot

• I have added tests and executed them locally
• I have updated CHANGELOG.md
• I have updated documentation

Review Checklist

This section is for contributors to review your work.

• Accessibility reviewed (tab order, content readability, alt text, color contrast)
• Browser and platform compatibilities reviewed
• CSS styles reviewed (minimal rules, no z-index)
• Documents reviewed (docs, samples, live demo)
• Internationalization reviewed (strings, unit formatting)
• package.json and package-lock.json reviewed
• Security reviewed (no data URIs, check for nonce leak)
• Tests reviewed (coverage, legitimacy)

[corinagum]

LGTM! Approved, pending a few more comments.