Now that we have E2E tests, let's make sure the the most important WordPress Playground features are covered.

We'll know it's done, when these tasks are completed:

• E2E tests for GitHub integration #805
• E2E tests for Gutenberg PR Previewer #803
• E2E tests for WordPress PR previewer #804
• E2E tests for HTTP error handling #950
• Run E2E tests on Chromium and Firefox #808
• E2E test cache busting on playground.wordpress.net #855
• Run E2E tests on a built site, not on the dev server #809
• E2E tests for playground.wordpress.net #765
• E2E tests with cypress #802