Create sample unit tests for components

[brettdh]

split from #15

Motivation

We want to establish a pattern for testing that components produce
expected output based on the inputs they are given. We also want to
have a pattern for testing components that are connected to Redux
state.

The component/store seam is a useful place to validate behavior.
We have state-focused tests that ensure that dispatched actions update
the state in expected ways; this issue is about ensuring that

1. the components handle a given state in the expected way; and
2. the components dispatch actions in expected ways in response to interactions
   (e.g. the onPress handler of a button)

Plan of action

Create sample tests for these cases:

• Presentation-only test
  • Either:
    • Test a component that isn't connect()ed to Redux; or
    • Export a presentation component (the one you wrap with connect())
  • Pass it props and assert something based on the rendered DOM
• Test mapStateToProps/mapDispatchToProps directly
  • Pass them a state object / dispatch function
  • Assert on the resulting props object
  • For mapDispatchToProps, assert that the functions returned dispatch the expected action(s)