Test upstream-generated sites automatically

[acobster]

As developers, we want high confidence that any sites we spin up will have a baseline set of features ready to develop on top of and ship to production quickly. We can leverage Cypress and other testing tools to do this fairly simply in a continuous integration environment.

Acceptance Criteria

• Select a CI environment to run integrations in
• Select a minimal set of features to test (three or four to begin with). These could be user-facing, frontend functionality, like nav menus, or it could be tooling like running lando some-command and expecting a certain result. Anything that should be there on a brand new Grootifer project is fair game
• Write tests in Cypress, PHPUnit, Bats...whatever is appropriate
• Deploy tests to the sitecrafting/wordpress upstream repo in Bitbucket
• Set up a nightly job to spin up a real Pantheon site from the upstream and run the tests against it
• Tests can also be triggered manually