This makes -t sample-accurate for audio and will allow further
simplication in the future.
Most of the FATE changes are due to audio now being sample accurate. In
some cases a video frame was incorrectly passed with the old code, while
its was over the limit.
Each fate-seek test depends now only on the corresponding fate-acodec,
fate-vsynth2 or fate-lavf test which creates the file seek-tests
operates on. The tests and references are renamed to match the test they
depend on.