Add unit tests

[smallketchup82]

Quoted from the discord:

i want to investigate creating unit tests to test certain aspects of KetchupBot. im not entirely sure as to how useful this will be, but at the very least it would help us check if ketchupbot properly formats, updates, merges, and sanitizes data. we could use jest and ts-jest for this

[smallketchup82]

again, i'm still not too sure as to what i should necessarily do in unit tests, or what to test, for that matter

i guess i'll write some tests that investigate the end result of an update, and the individual components. idk

[smallketchup82]

i can see #102 being a prerequisite to this, but i can unironically access the variables even though ketchupbot-updater isn't a class library

i could get started on defining tests, but it might just be better to address #102 beforehand, to keep things structured