[ticket/12620] Fix functional tests

PHPBB3-12620

phpBB/includes/functions_install.php

@@ -472,6 +472,7 @@ function phpbb_create_config_file_data($data, $dbms, $debug = false, $debug_cont
 	if ($debug_test)
 	{
 		$config_data .= "@define('DEBUG_TEST', true);\n";
+		$config_data .= "@define('DEBUG', true);\n"; // Mandatory for the functional tests, will be removed by PHPBB3-12623
 	}
 
 	return $config_data;