tests/lavfi: rework lavfi-regression.sh, avoid use of ffmpeg -pix_fmts

Use lavfi-showfiltfmts for checking the format supported by the scale
filter instead. The advantage is that the scale filter may support
formats which are not supported by libswscale (namely: the pal8 format).

The new implementation is also a bit cleaner, because it doesn't depend
on the ffmpeg output.
