I could find many questions/answers on why a method should not make it public. but I could not find anything specific to "default" in Java.
'default', i.e., the absence of a modifier, means package private in Java. Only classes in the same package can access it. Sometimes it is desirable to test an internal method meant for private use of that class in a unit test separate from the rest of that class in order to cover all code paths with clear, concise, and simple tests. When you do this (and the result is cleaner test code that can be maintained with more ease) it is fine to mark that method as package private.
This is not an uncommon strategy. Because the only classes who can use this method must reside in the same package, you still have plenty of control over its use.
Personally I would recommend only doing this for static utility methods that do not depend on the state of their parent class. It is also a very useful technique for testing static methods in abstract classes.
Note that in some cases the need to test private methods may point to the need to break out that part of the class into a separate class instead. This discussion shows some of the common standpoints varying from strict OOP adherence to pragmatism. You often have the possibility to break out that method and make it into a proper public static utility, but that doesn't always make sense, and it doesn't necessarily lead to easier maintained code.