Add axe-core automated accessibility tests

[afercia]

Trac ticket: https://core.trac.wordpress.org/ticket/66015

Proof Of Concept (POF) to add axe-core accessibility tests for the admin pages.

For now, this POF only scans one page.
Re-introduces the bug fixed in https://core.trac.wordpress.org/changeset/62851 for testing purposes.

• Make sure your local environment is correctly set up to run the E2E tests.
• Run npm install and then build.
• IMPORTANT NOTE: running the e2e tests will delete all your posts. Make a backup first.
• Run: npm run test:e2e -- tests/e2e/specs/accessibility.spec.js
• Observe the test reports 1 violation (the one intentionally re-introduced as mentioned above).
• Observe the output:

--- Violation #1 ---
Rule ID:   select-name
Impact:    CRITICAL
Failure:   Ensure select element has an accessible name
Help:      Select element must have an accessible name
Help link: https://dequeuniversity.com/rules/axe/4.13/select-name?application=playwright
Failing elements:
  - Target Selector: #media-attachment-date-filters
  - HTML Snippet:
<select id="media-attachment-date-filters" class="attachment-filters"><option value="all">All dates</option></select>

Axe documentation:

@axe-core/playwright documentation:
https://github.com/dequelabs/axe-core-npm/blob/develop/packages/playwright/README.md

Axe JavaScript Accessibility API
https://github.com/dequelabs/axe-core/blob/master/doc/API.md

Use of AI Tools

AI assistance: Yes
Tool(s): GitHub Copilot
Model(s): Claude Haiku 4.5
Used for: test suggestions; final implementation and tests were reviewed and edited by me.

---

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Core Committers: Use this line as a base for the props when committing in SVN:

Props afercia, joedolson, sabernhardt.

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[afercia]

Note that axe-core will report the group role added to the list item in the admin bar as a violation. Looks liek that's a known false positive. See dequelabs/axe-core#4910 and dequelabs/axe-core#3439

[joedolson]

Yes, that's well known; it's been reported against core a few times already. (Searched for the tickets, but didn't find them immediately.)

[sabernhardt]

I found one report on Trac 61795.

[afercia]

Update: the axe-core rule aria-allowed-role runs only if the configuration allows to run the best-practice set of rules. In fact, this is a separate group of rules as aria-allowed-role isn't strictly required by the WCAG. We should make a decision on whether to run the best-practice rules as they may add many reported violations. Maybe we could introduce a flag to run them optionally.