Stub the BeforeUnloadEvent.prototype.preventDefault Method

If the application shows the "Leave page" popup using the "e.preventDefault()" method call in the "beforeunload" event handler, we can confirm it in the Cypress tests using a stub on the "BeforeUnloadEvent.prototype.preventDefault". This video shows the solution, for more read the blog post "window.onbeforeunload and Cypress" at https://glebbahmutov.com/blog/onbeforeunload/