E2E tests for Gutenberg PR Previewer

[adamziel]

Let's create a few E2E tests for the Gutenberg PR previewer at https://playground.wordpress.net/gutenberg.html

Technically, it's difficult to hardcode a PR number with a well-known artifact since GitHub removes them after a while. However, Cypress can mock network requests which means we can reliably ship a ZIP file with the test suite and make sure the tests always use that.

Related:

• E2E tests with cypress #802
• E2E tests for playground.wordpress.net #765

[adamziel]

Gutenberg PR previewer was reduced to a ?gutenberg-pr Query API parameter supported by Playground core and will be reduced further to a github:artifact resource type. Let's address the testing there.