rename test to indicate that it came from mozilla
take buffer-loader.js from http://www.html5rocks.com/en/tutorials/webaudio/intro/
Port a gain node test from the webkit test suite.

I've reused Ehsan's buffer comparison function from the mozilla tests
for compatability, and removed the webkit's automatic comparison of
.wav files. Otherwise the setup and comparison works as in the webkit
source. I load the expected wav file using buffer-load.js and compare
it channel-by-channel to the rendered buffer from the
offlineAudioContext.