Closes #{45}
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I've added the base set of unit tests that this project should have (in my opinion :) ). I have added Enzyme as a dependency and hooked that up to the project in `src/setupTests.js` (as this is what CRA requires).
I've also removed constructors from the components as the function binding doesn't need to happen and you can assign a default state using the `state = {}...` syntax at a class level, rather than constructor level.
Let me know if anything else needs to be done for this PR.