Examples for unittest module

[JosXa]

This should be a simple first issue for someone who has experience with the stdlib unittest module, perfectly suited for Hacktoberfest 🥰

There is already a pytest suite in the examples folder. It would be nice to also include a sample test suite for the builtin Python unittest module for devs who are more familiar with that.

I have no idea how asyncio works in combination with unittest, but that is obviously a requirement.
What I'm expecting would be that you pick any bot (it can even be your own if you're sure it's going to stay online a high percentage of time) and create some class-based and function-based tests for basic bot functionality based on what you know are good testing practices.