Add pytest tests

[mattjegan]

At the moment we don't really have proper tests so it would be nice to have pytest working and full unit tests implemented.

[cat-turner]

hello, I'd like to help on this task. What tests are you thinking about?

[mattjegan]

Hey, thanks for taking this one. We'd be looking at creating a new tests.py file that would import the different classes and functions in HowLong.py and try to cover as many lines as we can. This hopefully will help to discover some bugs along the way.