add snapshot testing for all components

Currently there are no tests, although the testing support is there from the Create React App boilerplate.

We should aim to at least have snapshot testing since it yields the most value for the time invested.

Start by locating the Create React App testing support and the component entry points; the issue names no files or existing tests. Determine the complete component set and how snapshots should run, with the work done when all components have passing snapshot coverage.

Written by the indexing model from the issue text.