Marker interface for methods used by test code in the same module. Methods/Constructors annotated
with this method should not be accessed in production code. This annotation should be used
sparingly as it's a code smell to need access to internal data/functionality to properly unit
test a class. Typically there is a better way to test a class.
TODO: Write a linter that makes sure only test code depends on methods or constructors annotated
with this method