Write unit tests

[lourot]

[killerkc12]

@AurelienLourot If this issue is still open, can I pick this up?

[lourot]

Hey @killerkc12 👋 Yes this is still open, thanks a lot! Let me know if you have any questions

[killerkc12]

@AurelienLourot I was going through the codebase.
I've seen test script is explicitly looking for jest.config.json, can we directly use the jest package here?
Also if we use the jest, it will directly use the jest.config.js (will create a new file) where we can use the global config.
If you're ok, I can give a PR