write a test for initCanvas

[ericblade]

... test for initCanvas needs to test that it correctly sets Quagga.canvas.dom and Quagga.canvas.ctx, and that those each have an image and an overlay property.

Per #151 once implemented, if you don't have a DOM drawingBuffer then Quagga.canvas.dom.overlay and Quagga.canvas.ctx.overlay should be nullish, there should always be a Quagga.canvas.dom.image and Quagga.canvas.ctx.image when in browser. in node, they should all be nullish.