Simpletest, Drush 8, test-run, and Exit Codes.
If you're using Drush to run your tests, swapping Drush version 6.x out for 8 isn't a simple replacement. First drush test-run has been removed so running Drupal simpletests now requires using the run-tests.sh. This would be workable, however under Drupal 7, scripts/run-tests.sh does not return correct exit codes so if you're relying on failed tests returning a 1 you are out of luck. While this has been fixed for Drupal 8, if you're running Drupal 7 you're going to have to go an alternate route.