If you've followed Pantheon's Build Tool Instructions and used Pantheon's Example Drops 8 Composer as the base to take advantage of CircleCI then you've encountered Behat tests. Behat tests are extremely valuable for testing site functionality before new code goes to production or a shared code stream. However, on a custom site these default tests provided by Pantheon are likely to fail if you've made even a benign change like deleting Tags vocabulary on a Drupal standard installation.