I agree very much with this. I'd add one thing, adding tests and testing your code are not the same thing. You should write tests, mostly integration tests, testing public well defined boundaries of your class, your component, your service. Mock only IO if needed, but not other classes. But also tests the rest, but no need to add a test for them. Just run the code, try out the private functions, make sure they work.

And also go read Testivus: https://www.artima.com/weblogs/viewpost.jsp?thread=204677