Ask HN: What is the best testing strategy?

In the making of a new language, i was wondering, what testing strategies are out there? Which one(s) is/are the best? And why?

Follow up, What is it exactly that causes us to need tests? To me, it sounds like, the compiler just cannot understand all code. E.g. when writing data to a pointer, the compiler does not know how much space is allocated for that pointer. Is it possible to make the compiler understand, or is that crazy work?