Add tests for core logic

[kas-catholic]

It would be good to have some tests around the logic to add and remove items. These should be built in whatever framework is most commonly used by create-react-app.

[abhimaurya-dev]

I would like to work on adding tests. I am thinking about using Mocha framework. Do you have any recommendation of what framework i should be using ?

[kas-catholic]

We do already have some tests (written since this issue was created, I think). They can be run with yarn test. They're using Jest, so we should continue using that framework.

E.x. https://github.com/kas-catholic/confessit-web/blob/ea1e2c06e7ac7c87d7ae355d27e24c5b541fa3fe/src/AddButton.test.js#L38-L45