Digital Twin (DT) is one of the pillars of modern information technologies that plays an important role on industry's digitalization. A DT is composed of a real physical object, a virtual abstraction of the object and a bidirectional data flow between the physical and virtual components. This paper presents a DT-based tool, called TaS, to easily test and simulate IoT environments. The objective is to improve the testing methodologies in IoT systems to evaluate the possible impact of it on the physical world. We provide the conditions to test, predict errors and stress application depending on hardware, software and real world physical process. The tool is based on the DT concept in order to detect and predict failures in evolving IoT environments. In particular, the way to prepare the DT to support fault injection and cybersecurity threats is analyzed. The TaS tool is tested through an industrial case study, the Intelligent Transport System (ITS) provided by the INDRA company. Results of experiments are presented that show that our DT is closely linked to the real world.