Add tests to the `tests` module

[DanielCohenHillel]

The tests module is pretty empty right now, we only test for very few things... Anyone who wants can add more tests, all you need is use case for the code and a way to check the results. If you want to give it a go, check out the tests folder. Good luck :tada:

If you do plan to contribute, please edit the develop branch and not master (use git to checkout to develop and do your edits there), it's not the end of the world if you commit directly to master but then it would be hard reviewing the code before uploading it for everyone to use.

[zlatko-minev]

This of very good to do indeed