Improve Test Performance

[n7studios]

Summary

Reduces end-to-end test suite runtime, with no change to Plugin functionality.

Tests: 676 → 644

Removed tests that duplicate coverage elsewhere:

• Cache/optimisation Plugin tests existed in four Cests. The block and shortcode versions both cover an inline Form; the Block Editor and Classic Editor versions both cover a modal Form. Removed the duplicate of each pair, keeping PageBlockFormPerformancePluginsCest (inline) and BlockEditorFormCest (modal).
• Default Form position tests in ClassicEditorFormCest duplicated BlockEditorFormCest, as the position logic runs on the_content and is editor independent. Kept testAddNewPostTypeUsingDefaultFormAfterParagraphElement, as the Classic Editor's content is generated by wpautop() and the Block Editor's by do_blocks(), which can produce a different number of elements.
• Refresh button tests for the Form Trigger and Product blocks/shortcodes duplicated the Form ones. Kept one block, one Visual Editor modal and one Text Editor modal.
• ImportWhenNoKitForms existed in all seven importer Cests for the same shared code path. Kept on PluginSettingsToolsImporterActiveCampaignCest.

24 shortcode tests now assert the shortcode built by the Visual or Text Editor modal and stop there, instead of also publishing and viewing the Page. Both modals assert the same expected shortcode, and the block and shortcode share the same render() method, so the frontend output is already covered by the block's test. Adds clearClassicEditorUnsavedChangesWarning(), which clears WordPress' beforeunload handlers so navigating away from the unpublished Page doesn't display a browser dialog.

GitHub Actions

• Each test group now installs only the third party Plugins and Themes its tests activate, defined in the matrix, instead of every job installing all of them. Divi 5 is now only installed for integrations/divi-theme, and uncode and impeka only for restrict-content/post-types.
• wordpress-seo and wpforms-lite are no longer installed, as no test activates them.
• Added a build-assets job that runs npm ci && npm run build once and uploads the built CSS and JS. Test jobs download that artifact instead of running npm.
• composer update is now composer install, so every job installs the versions in composer.lock.
• npm run build no longer runs the CSS and JS linters, which were running in every test job. Linting is now npm run lint, and runs in the Coding Standards workflow only.

Testing

Existing tests pass.

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)