In isolation it surely makes sense to have tests for the behaviour. But given finite time it makes more sense to test from the perspective of what the software brings value to instead of the details. Like this char_value function will be used somewhere. Depending on how important it is to that user testing will include what char_value does.
Then there are layers in software. Just like with third party dependencies you can have first party dependencies by having modules in your codebase sitting at lower levels of abstraction tested in isolation to the higher levels - e.g. to keep test complexity down. That‘ll be different modules though, which can have public functions. If you‘re looking for that I‘d strongly consider not leaving them inline with the higher level abstraction code.
