Add end-to-end (e2e) tests

[swissspidy]

We already have some e2e tests for the WP-CLI entry point, but not for the admin entry point. Having e2e coverage for the Ajax flow would increase confidence in our setup.

Should be simple to set up, with 1 dummy plugin for testing and 1 addon installed.

Can use Playwright and wp-env or maybe even Playground

Scenarios to test:

• Running all checks and their output
• Running only runtime checks
• Running checks from an add-on
• Maybe more?

[felixarntz]

Oops, I just noticed that I accidentally had marked #751 to fix this - was somehow confusing Behat and e2e 😆