Tests

[leeoniya]

it's becoming more time-consuming and error-prone to manually re-test all the demos following internal refactorings and API adjustments.

now that the API is fleshed out a bit, it's possible to test a large amount of code (non-granularly) without having to simulate all interactions via Puppeteer or similar.

a lot of code can already be regression-tested by simply running all the demos and validating the .outerHTML of the dom, and diffing reference images of resulting uPlot canvases via something like https://github.com/mapbox/pixelmatch.