Create a test for the function with testthat

[BatoolMM]

This is not a priority and doesn’t need to be completed before we make the package public.

It's to add a unit test to the function so that when optimised, we can test it.

@RayStick feel free to remove the issue if you think it's not needed at all or has been mentioned in another issue that I missed.

[RayStick]

I think unit testing is a great idea (eventually)