Backend unit tests

From Full-Stack-Project-Assessment created by [Dedekind561](https://github.com/Dedekind561): CodeYourFuture/Full-Stack-Project-Assessment#482  

## Acceptance criteria

- [ ] All backend tests in `api.test.js` are passing

## Guidance 🧭 

Check out the [testing guide](https://github.com/CodeYourFuture/Full-Stack-Project-Assessment/tree/main/guides/testing) to setup your backend unit tests

