Add lavfi-pixfmts LE tests.

The corresponding lavfi-pixfmts BE tests are not yet added, as there
are some bugs in the scaler (scaling rgba, argb, bgra, abgr, yuva420p)
which result in differences with the LE reference, and I cannot
visually check the generated files on BE.

Originally committed as revision 24657 to svn://svn.ffmpeg.org/ffmpeg/trunk
