When I was doing safety critical software at the beginning of my career, we had a whole team in charge of building simulators replicating the API provided by th

When I was doing safety critical software at the beginning of my career, we had a whole team in charge of building simulators replicating the API provided by the pieces of equipment we were integrating and their full behaviour. These simulators were themselves fully tested using both mock network traffic and recording of the real piece of equipment. This was used for integration testing and then repurposed for training. Every testing environment I have seen since have looked like a joke but to be fair the testing budget of this place was bigger than what some companies spend on a whole product.