I've created a helper function for my own purposes and thought it would be useful to others ...
As the Monastery has taught me that all published modules should have tests,
I want to do it probably and provide those tests ... what should I test?
Bod, you are asking this question too late! The Monastery has also taught you to write the tests first
because the act of writing your tests changes and improves your module's design:
Writing a test first forces you to focus on interface - from the point of view of the user.
Hard to test code is often hard to use. Simpler interfaces are easier to test.
Functions that are encapsulated and easy to test are easy to reuse.
Components that are easy to mock are usually more flexible/extensible.
Testing components in isolation ensures they can be understood in isolation and promotes low coupling/high cohesion.
Implementing only what is required to pass your tests helps prevent over-engineering.
-- from "Test Driven Development" section at Effective Automated Testing