tests: Bypass native ariaNotify in polyfill tests

[smockle]

Problem

Native support for ariaNotify shipped in Firefox 150.0: https://www.firefox.com/en-US/firefox/150.0/releasenotes/. As a result, our existing tests—running in Firefox—no longer test the polyfill itself.

Solution

• Support loading the ariaNotify polyfill even in browsers with native ariaNotify support, to enable polyfill testing (both Web Test Runner and Guidepup).
• Use Chrome for browser-backed tests because it is preinstalled on GitHub Actions runners and Web Test Runner’s default. This reduces configuration, browser installation time, and failure points.

[Copilot]

Pull request overview

This PR updates the test harness to ensure automated tests continue exercising the ariaNotify polyfill even in browsers that ship a native ariaNotify, and aligns browser-backed tests around Chrome/Chromium defaults.

Changes:

• Inject a test-only “bypass native ariaNotify” shim into Web Test Runner HTML tests and Guidepup Playwright contexts.
• Update the polyfill to optionally override native ariaNotify when a test-only global bypass flag is set.
• Switch Playwright projects to run against Google Chrome (channel) and simplify test scripts by removing explicit Playwright browser installs.

Show a summary per file

File	Description
web-test-runner.config.js	Injects the bypass shim and polyfill into WTR HTML tests; removes explicit Firefox Playwright launcher setup.
tests/guidepup/voiceover.spec.mjs	Adds a shared init script to bypass native ariaNotify for macOS VoiceOver tests.
tests/guidepup/nvda.spec.mjs	Adds a shared init script to bypass native ariaNotify for Windows NVDA tests.
tests/bypass-native-arianotify.js	New test-only shim that forces failures if tests hit native ariaNotify and sets a bypass flag for the polyfill.
playwright.config.mjs	Switches Playwright project to Google Chrome (channel).
package.json	Removes explicit playwright install firefox from test scripts.
arianotify-polyfill.js	Adds support for a test-only bypass flag and uses defineProperty to override native ariaNotify when requested.

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• Files reviewed: 8/8 changed files
• Comments generated: 1

[JoyceZhu]

Looks great!