This commit starts to convert the tests to a system where the codec
to be used is provided by a factory object. Currently no tests are
instantiated for VP9 since they all fail for various reasons, but it
was verified that they're called and the correct codec is
instantiated.
Change-Id: Ia7506df2ca3a7651218ba3ca560634f08c9fbdeb
Exlcude key frame from buffer underrun check, and increase
lowest bitrate in BasicBufferModel.
Both changes are needed because of a known issue (#495).
Change-Id: If5e994f813d7d5ae870c1a72be404c8f7dbbdf27