Here's a quick post to show how we can run Drupal in a CI environment easily so that we can test the site. Regardless of how you choose to run the tests (e.g. PHPUnit, Behat, etc), you still need to run the site somewhere. It is not a great idea to test on an actual environment (unless it is isolated and designated for testing). You need to set up a temporary environment just for the CI pipeline where you run the tests and then tear it down.