When 12775 was merged, it introduced a change that cleared out $dbpasswd:
https://github.com/phpbb/phpbb/pull/2671/files#diff-a44c6a7fa1f1be1b78f4f8255491ef20R151

That variable is needed a few lines down to build the connection configuration array. As it stands now, test_config.php files cannot be used to run the test suite.