test.html: Refactor zoom slider ids.
test.html: Refactor zoom slider ids.

In preparation of adding pan/tilt sliders, give more logical names to
zoom slider.

davidbrenner/vb-c10-network-camera-js-client,davidbrenner/vb-c10-network-camera-js-client,davidbrenner/vb-c10-network-camera-js-client