Fix constant already defined error when running only Cache tests.

tests/bootstrap.php

@@ -19,7 +19,9 @@ use Cake\Log\Log;
 
 require_once 'vendor/autoload.php';
 
-define('DS', DIRECTORY_SEPARATOR);
+if (!defined('DS')) {
+	define('DS', DIRECTORY_SEPARATOR);
+}
 define('ROOT', dirname(__DIR__));
 define('APP_DIR', 'TestApp');