unit tests

Hi! I integrated `useHasOverflow` in our TSX app and it helped me a lot, so first of all thanks for that.
Now I will have to write tests for it since we want all used code to be test-covered, which also goes for dependencies.
I would like to give back to this repo and submit the test code.

So my questions are:
- are you open for a PR here?
- are you planning to add a testing setup any time soon? Prep work here would keep the scope of that eventual PR as small as possible