This is amusingly naive. I've seen the spec for Tetris, and it's surprisingly big - much larger than the smallest known fully conforming code, which is still surprisingly large. There are also a non tiny number of people knocking around for whom it's their entire job to test it. "Even" Tetris? No, sorry - that's nonsense.

The big problem here is that code coverage tests don't help you cover what you should have explicitly defined or tested but didn't. As a result a lot of things end up still defined by implementation and not specification, as all sorts of important details only got defined during implementation.