(don't merge yet) Testing

@maxmurdoch @harrison a first start at testing. Lots more to write obviously but thought I'd kick off the discussion on what we do or don't want to write about.
‘stubbed in the test.’

‘Inject the dependencies and the components being tested in a `beforeEach` function. This encapsulates each test’s state, negating the need to run them in a specific order, and making them easier to reason about.’