Improve integration tests for live AudioContext

[orottier]

Most of our integration tests use the OfflineAudioContext which makes it very easy to inspect the generated samples.
Not everything can be tested offline though, e.g. the MediaElementSourceNode, progression of time, events, etc.
We need a method for testing live AudioContexts.

Perhaps we can introduce a TestBackend (similar to NoneBackend) which

• generates a fixed number of frames and runs faster than real-time (like the OfflineAudioContext)
• can be polled for the audio output samples
• is only exposed in cfg(test)