The usual convention is to split your module into public and private parts, i.e.
module SomeModule.Internal where
-- ... exports all private methods
and then the public API
module SomeModule where (export1, export2)
import SomeModule.Internal
Then you can import SomeModule.Internal in tests and other places where its crucial to get access to the internal implementation.
The idea is that the users of your library never accidentally call the private API, but they can use it if the know what they are doing (debugging etc.). This greatly increases the usability of you library compared to forcibly hiding the private API.