Fill out tests in waveform and waveform_backend

[sabjorn]

We have chained functions (via fetch and offlineAudioContext) which are not really tested in the code. The "was this function called" code exists BUT the functions that are called as a result are not tested.

This really isn't a high priority because the waveform obviously works. However, it's just not great to rely on this sort of visual check.

[sabjorn]

examples of testing that might be helpful here:

• this one shows how to return a promise with a stub
• this shows how to get a function that is passed as an argument to a stub in order to test it