Remove payload duplication in AudioDecoderTest

This hack was made to come around issue 845. Now that is solved, and
the test code can be cleaned up.

BUG=845
R=kwiberg@webrtc.org, tina.legrand@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/21709004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6534 4adac7df-926f-26a2-2b94-8c16560cd09d