Create tests for rendered components

[mhkeller]

Currently, the library's JavaScript modules and utilities have tests but we don't have anything that does end-to-end testing of rendered components. This has been fine since the library hasn't changed much over the last few years. But, making sure that the changes in #179 are working properly and also anticipating a full rewrite to using runes, it would be good to have some benchmarks.

I've been looking into some ways to test Svelte components and it seems a bit complicated. I'm thinking the best way to do it would be to look at the LayerCake context for each of the examples on the website and make sure it matches the expected data. Open to ideas and what other people have done in this space.