crashtest for bug 933156

--HG--
extra : transplant_source : %9D/%19%13%80%D3cY%5D%93%FA%17%FB%CFTO%5Ex%82%C9
b=936317 prevent MediaBufferDecoder threads from waiting after AudioContext shutdown() r=ehsan

--HG--
extra : transplant_source : %CE%E5O%83N%8FX%ED%2Ck%17I%B1%92%B6%A0lXH%D1
b=931311 wait for HRTF panner to load impulse database before doing test

--HG--
extra : transplant_source : %2A%1F%D0kKSr%AA%8F%C1%26%E5%E6%EC%D5%CF%8C%A9%D11