Add libavfilter 1-input - 1-output regression test, corresponding to the

target regtest-lavfi_pix_fmts.

The lavfi_pix_fmts test is disabled, this because there are
many tests which are failing, and there are still some output files
which cannot be played by NUT/ffplay.

Originally committed as revision 23297 to svn://svn.ffmpeg.org/ffmpeg/trunk