Run tests in headless browser framework?

[adroitwhiz]

Expected Behavior...

...should be easy to verify via tests.

Actual Behavior

...could differ significantly between Node + JSDOM, which is where tests are currently run, and the browser, which is where the SVG renderer is used.

The SVG renderer uses a significant number of DOM functions which are out of the scope of what JSDOM emulates-- for instance, the SvgRenderer constructor calls HTMLCanvasElement.getContext, and measuring an SVG requires calling getBBox.

In order to test those parts of the API, those functions must work properly, which requires running tests inside a browser.

I ran into this when writing tests for #85, which required a lot of shimming.

Adding in-browser testing support may also help with #119.