[Test] Add e2e test on beforeunload bug

[christian-byrne]

This PR adds a test on the issue fixed in Comfy-Org/ComfyUI_frontend#2318 by creating an unsaved workflow and then closing the page with runBeforeUnload option enabled.

Additional Changes:

• Switch to using Playwright Electron
• Before each test, delete existing config file in order to simulate new user
• After each test, teardown electron process and browser
• Add fixtures

Running with npx playwright test --ui:

electron-playwright.mp4

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