I released version 1.0.0 of a small utility library with Cypress commands that can help you write tests of an OpenID Connect Provider. Check it out here: https://www.npmjs.com/package/@curity/cypress-commands 

At Curity we use it to write end-to-end tests of our open-source plugins. E.g. this authentication action plugin uses this library for tests: https://github.com/curityio/redirect-action-example