Define PIWIK_DOCUMENT_ROOT constant for tests.

tests/PHPUnit/bootstrap.php

@@ -11,6 +11,9 @@ define('PIWIK_TEST_MODE', true);
 if (!defined("PIWIK_PATH_TEST_TO_ROOT")) {
     define('PIWIK_PATH_TEST_TO_ROOT', realpath(dirname(__FILE__) . '/../..'));
 }
+if (!defined('PIWIK_DOCUMENT_ROOT')) {
+    define('PIWIK_DOCUMENT_ROOT', PIWIK_PATH_TEST_TO_ROOT);
+}
 if (!defined('PIWIK_USER_PATH')) {
     define('PIWIK_USER_PATH', PIWIK_PATH_TEST_TO_ROOT);
 }