Change PDO to singleton instance

- Add SQLite throwaway DB implementation
- Add integration tests for Custom class

src/Configuration.php

@@ -58,4 +58,10 @@ class Configuration
 
         return self::$_pdo;
     }
+
+
+    public static function resetPDO(): void
+    {
+        self::$_pdo = null;
+    }
 }