Add simple tests

There should be have some simple tests for the functionality of the app as a whole (not just "Assert `returnOne()` == 1"

This is a bit of a brainstorming operation. What functionality do we need to ensure works? But also, what functionality is more subtle such that we won't know if it is operating correctly at first glance? The test framework is set up using Jest, but there is only one "dummy" test at present.

- [ ] Research Jest library
- Where should I and should I not use 'mocks'?
- [Jest](https://jestjs.io/docs/getting-started)
- [ ] Redux tests?
- [Redux - Writing Tests](https://redux.js.org/usage/writing-tests)