Why is the default access modifier in JUnit 5 package-private?
Tests in JUnit 4 had to be public.
What is the benefit of changing it to package-private?
Why is the default access modifier in JUnit 5 package-private?
It's not the "default". There technically is no default. Rather, in JUnit Jupiter you have a choice: public, protected or package-private.
What is the benefit of changing it to package-private?
The benefit is that you don't have type public anymore. If your IDE automatically generates test methods and test classes for you that are public, feel free to leave them public.
But... if you are typing in the methods on your own, then just leave off public unless you are designing your test classes for subclassing from other packages, in which case you'd want to make your overrideable test methods either public or protected. And of course, interface default methods must be public.
Long story, short: we (the JUnit 5 team) believe in the principle "Less is more", meaning the less you have to type to achieve your goal, the better!