Add common FFTFrame and decodeAudioData to tests.

Summary:
MediaStreamAudio and WebAudio will shared the FFTFrame and
decodeAudioData stubs. These are needed to make the test
work in debug mode.

Reviewers: Tullio, lacerda, dakerfp, sergio, luck, setanta, nick, zanoni, hugopl

Differential Revision: http://phabricator.coderev.indt.org/D3128