Add E2E tests for Axe plugin #1224

User story

Add E2E test coverage for the setupScript option to verify the Axe plugin works correctly when testing login-protected pages.

The existing E2E test in e2e/plugin-axe-e2e only covers basic usage with a static HTML file:

• mocks/fixtures/index.html - static HTML page
• mocks/fixtures/code-pushup.config.ts - config using axePlugin(url) without options

Acceptance criteria

• Add E2E tests for the Axe plugin running against a local example app
• Include a test scenario that validates the setupScript option for authentication
  • Test should authenticate against a login-protected page before running accessibility scans
  • Verify that audits are correctly reported for authenticated content

Implementation details

1. Create a mock authenticated app fixture

Add a new fixture directory e2e/plugin-axe-e2e/mocks/fixtures/auth-setup/ including:

• simple Express server with:
  • login page with username/password form
  • protected page that redirects to login if not authenticated
  • session handling via cookies
• Playwright setup script
• code-pushup config file using setupScript option

2. Add a new E2E test case that verifies report.json contains audits for the authenticated page