Even Better Tests!

[MahdiBM]

Problem

Penny doesn't have enough unit-tests, which means we'll have to test more in production.
Testing in production means we'll be disrupting users that are expecting the bot to work, which is not good.

What to do?

We need to have good-enough unit-tests so we can trust Penny to be working properly after an update.

How?

In #21 we added all required abstractions that are needed for testability.
Now, writing tests is only a matter of writing some code in the test targets, and won't require much changes elsewhere.