Are we not already more or less there? It is not perfect, to be sure, but LLMs will get you pretty close if you have the documentation to validate what it produces. However, I'm not sure that removes the tedium the parent speaks of when writing tests. Testing is not widely done because it is not particularly fun having to think through the solution up front. As the parent alludes to, many developers want to noodle around with their ideas in the implementation, having no particular focus on what they want to accomplish until they are already in the thick of it.

Mind you, when you treat the implementation as the documentation, it questions what you need testing for?