Tests: Add visual regression tests for admin reskin pages

[josephfusco]

Summary

• Adds 10 new visual regression snapshot tests covering admin pages most impacted by the CSS reskin in Feature: Admin Reskin 7.0 #10910 that previously had no automated visual coverage
• Combined with the 22 existing tests, this brings total coverage to 32 admin pages
• Adds a single-command workflow (npm run test:visual) that automates baseline generation from trunk and comparison against the current branch
• The admin reskin is a CSS-only change touching 27 files — visual regression tests directly test what changed

New tests

Test	Page	Notes
Dashboard	/index.php	First page every admin sees
Themes	/themes.php	Masks theme screenshot images
General Settings	/options-general.php	Primary settings form
Writing Settings	/options-writing.php	Form fields, checkboxes, dropdowns
Permalink Settings	/options-permalink.php	Radio buttons, text inputs
Add New Post	/post-new.php	Masks editor content area
Edit Post	/post.php?post=<id>&action=edit	Creates post via REST API, masks editor
Site Health	/site-health.php	Masks health check accordion
Updates	/update-core.php	Masks upgrade form, last-checked timestamp
Login	/wp-login.php	Unauthenticated, separate test describe block

How to use

From a feature branch with a clean working tree, start the environment and run:

npm run env:start
npm run test:visual

This will automatically:

1. Check out trunk to generate baseline snapshots
2. Return to your feature branch
3. Compare against those baselines

If any tests fail, diff images can be found in artifacts/.

To compare against a branch other than trunk:

npm run test:visual -- some-other-branch

Snapshots are gitignored — no storage overhead in the repo.

Trac ticket

https://core.trac.wordpress.org/ticket/64308

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