When you simplified the function, did you simplify away the inputs and outputs? While it is often possible to test functions that accept no inputs (depending on reading files, querying properties of objects in a GUI, etc. to control its behavior) and return no outputs but some change to the environment (creating a figure, writing a file, etc.) IMO it's easier if you can control the behavior via input arguments and validate the output arguments directly.
As an example, if the function you're testing must not accept inputs but instead be controlled via files that it reads in, you could still test it. Using the four phase test terminology, creating the control files corresponding to the behavior you want to test would be your Setup. Running the function you're testing would be Exercise. Checking that the artifacts created by the function match your expectations is Verify. Deleting the files that you created during Setup would be your Teardown, restoring the system (the function + the space on disk where the function looks for its control files) to its original state.
If you were able to provide a few more details, such as a less simplified version of your function, we may be able to offer more specific suggestions.