Like in software engineering, the auto industry needs to automate the tests. This Brazilian video shows all the Ford Corcel components being tested. Note that the tests don't finish on the labs. They put the car on the different streets putting the lab tests in pratice and, like in software engineering, the tests ends only in the customer environment. The video shows integrated tests and unit tests, in the labs and in the different real environments (even in the water).
To test a product you need to stress the parts, trying to broke the product, simulating the worst conditions. If you do it, and the product pass it, you had a good job.