Integrate Puppeteer with Github CI

I'm starting to learn about e2e testing and I've decided on using Puppeteer for its documentation and support. However, I'm having some trouble figuring out how to integrate with Github CI on specific branches. All examples online seem to render a webpage with a given URL, but obviously, that's not possible if you're trying to test a branch before deployment and merging. Other examples show rendering localhost:3000, so I'm wondering if its possible to start the page using npm start, and then run npm test. How should I approach this?