Adding test::AudioSink interface and derived classes

The AudioSink interface is supposed to be used by tests that produce
audio output. Two implementation classes are also provided:

OutputAudioFile: Writes the audio to a pcm file.
AudioChecksum: Calculates the MD5 checksum of the audio.

These will both be used in future changes.

R=kwiberg@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/17729004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6490 4adac7df-926f-26a2-2b94-8c16560cd09d