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.

This partially reverts acb17302
which would only have needed to change the checksums if channel mixing had
been properly avoided. This changes the output file size reference and the
seek test reference back to the previous values.

Change some lavf tests to avoid resampling and channel mixing.

Originally committed as revision 24345 to svn://svn.ffmpeg.org/ffmpeg/trunk

Originally committed as revision 22158 to svn://svn.ffmpeg.org/ffmpeg/trunk