You test components in isolation, you test integration of components, you run simulations of the entire rocket, and finally you test the rocket launch.

You’ll catch issues along the way, but you can’t catch all of them before a full launch test. That’s why there are launch tests.

This can get as far as the test plan is complete, multiply iterated under different interface conditions and thorough. And you are still relying upon the adherence of the simulated models to the physical reality.

Real tests do all of this at once with no option to escape reality.

Again, one thing is automating thorough software tests, another one is testing physical stuff.

This is the programmer fallacy if you have a bunch of code passing unit tests, it’s going to work when combined.