On a fresh installation of MediaWiki as of b347d8e where phpunit.php passed with no errors or failures, installing latest version of CheckUser as of 7a43255 and running the update script results in phpunit returning this error message:
1) PasswordResetTest::testExecute_email Wikimedia\Rdbms\DBQueryError: A database query error has occurred. Did you forget to run your application's database schema updater after upgrading? Query: INSERT INTO `unittest_cu_changes` (cuc_namespace,cuc_title,cuc_minor,cuc_user,cuc_user_text,cuc_actiontext,cuc_comment,cuc_this_oldid,cuc_last_oldid,cuc_type,cuc_timestamp,cuc_ip,cuc_ip_hex,cuc_xff,cuc_xff_hex,cuc_agent) VALUES ('2','','0',NULL,NULL,'reset password for user \"User1\"','','0','0','3','20170918004929','1.2.3.4','01020304','0',NULL,'0') Function: CheckUserHooks::updateCUPasswordResetData Error: 1048 Column 'cuc_user' cannot be null (localhost) /var/www/html/includes/libs/rdbms/database/Database.php:1149 /var/www/html/includes/libs/rdbms/database/Database.php:979 /var/www/html/includes/libs/rdbms/database/Database.php:1589 /var/www/html/extensions/CheckUser/CheckUser.hooks.php:124 /var/www/html/includes/Hooks.php:186 /var/www/html/includes/user/PasswordReset.php:203 /var/www/html/tests/phpunit/includes/user/PasswordResetTest.php:183 /var/www/html/maintenance/doMaintenance.php:92
This is because this line of code passes null for the second user, which works in MediaWiki core but fails when CheckUser is installed.