HTML API: Update html5lib-tests to web-platform-tests

[sirreal]

Update html5lib-tests to web-platform-tests.

The HTML API uses an external test suite to test how it processes HTML. That test suite has become part of web-platform-tests/wpt where it will continue to be maintained.

This also updates the test data to the latest version: web-platform-tests/wpt@a315c38

Notably, the external tests have added a processing instruction test suite. Processing instructions were treated as bogus comments in HTML but now produce processing instructions. This entire suite is currently skipped. The HTML API will be updated to handle Processing Instructions in another PR.

Test diff with trunk:

-WP_TESTS_SKIP_INSTALL=1 ./vendor/bin/phpunit --group html-api-html5lib-tests
+WP_TESTS_SKIP_INSTALL=1 ./vendor/bin/phpunit --group html-api-web-platform-tests
 # OK, but incomplete, skipped, or risky tests!
-# Tests: 1508, Assertions: 1088, Skipped: 420.
+# Tests: 1513, Assertions: 1091, Skipped: 422.

Trac ticket: https://core.trac.wordpress.org/ticket/65581

html5lib-tests have been taken over by web-platform-tests (WPT).

Update the HTML API tests to point to WPT instead of legacy html5lib-tests.

The updated test suite in WPT can be found here.

Use of AI Tools

AI assistance: Yes
Used for: Initial migration.

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

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[github-actions]

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[sirreal]

Note that the HTML API tests are running on CI with the updated --group.

TEST_GROUPS: html-api-web-platform-tests
…
Tests: 1513, Assertions: 1091, Skipped: 422.