public static function getConfigDir()
    {
        if (is_null(static::$configDir)) {
            $configDir = '';
            $dirs = [
                // path/to/module/test/sandbox/config directories
                __DIR__.'/../../../../*/sandbox/config',

                // path/to/yawik-standard/config
                __DIR__.'/../../../config',
            ];
            foreach ($dirs as $dir) {
                foreach (glob($dir) as $testDir) {
                    $configDir = realpath($testDir);
                    break;
                }
                if (is_dir($configDir)) {
                    break;
                }
            }

            if (!is_dir($configDir)) {
                throw new InvalidArgumentException('Can not determine which config directory to be used.');
            }

            static::$configDir = $configDir;
        }
        return static::$configDir;
    }