Comment by 9rx

Testing, which is better known as documentation, is the contract for users that defines what your program is for and how the user can depend on it forever into the future. The test runner is not the focus, rather the effort to validate that what the contract says is actually true.

Test what the user will use. That could mean an end-user interface, or it could mean a public API. Again, the goal is for the user to understand what you are trying to accomplish for them and to have guarantees about what you won't change in the future.