Add some tests for the app

[jerelmiller]

In a haste to get this demo up and running, I've ignored testing many aspects of this app.

Testing is a huge part of development on any app with Apollo Client. We want to be able to effectively demonstrate useful testing with Apollo Client. We should therefore be good citizens and provide some tests to demonstrate how we think about it.

Having some tests will also be useful when we are ready to execute on apollographql/apollo-client#9738 as we'll be able to see if our new approach feels right.