Extending testing to include snapshots

[heckj]

It's a bit early since there's nothing yet rendering, but one of the ideas I'd like to explore here is enabling rendering images of the charts from examples, capturing, and potentially comparing those images. At a minimum, I'd like to extend the DocTests target to render images of charts so that they could be captured and used in the DocC generated documentation showing explicit examples along with associated code snippets that generate them.

Leveraging PointFree's snapshot testing library seems like a potential starting point, but I haven't trod this road before, let alone with SwiftUI and how that may play together (or not).