This should not be a problem.
To test a private function the PL/SQL developer has to do the following things (automaticly):
1: Create an unique package identifier (UPID)
2: Copy the whole package to a new one named [UPID]
3: Locate a possible forward declaration of the function to test and comment it out.
4: Create a entry in the specification of the new package.
5: compile the package.
6: Start the test
7: After ending the test, remove package [UPID] from the Database.
It is not a one line-of-code change, but it's worth to do it, because we have to do those changes manually to test them.