Modifying NetEqExternalDecoderTest

This change is in preparation for extending the test with another
fixture that inherits from NetEqExternalDecoderTest.

R=tina.legrand@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/24609004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7213 4adac7df-926f-26a2-2b94-8c16560cd09d