$ drush test-run MyUnitTestCase
PHP Fatal error:  Call to undefined function module_exists() in ~/.composer/vendor/drush/drush/commands/core/drupal/environment_7.inc on line 187
Drush command terminated abnormally due to an unrecoverable error.                                                               [error]
Error: Call to undefined function module_exists() in
~/.composer/vendor/drush/drush/commands/core/drupal/environment_7.inc, line 187