lavfi-regression: provide filter arguments to showfiltfmts

In do_lavfi_pixfmts(), provide the filter arguments to showfiltfmts,
since some filter may require non-null or non-empty argument string
for working properly.

tests/lavfi-regression.sh

@@ -55,7 +55,7 @@ do_lavfi_pixfmts(){
     $ffmpeg -pix_fmts list 2>/dev/null | sed -ne '9,$p' | grep '^\..\.' | cut -d' ' -f2 | sort >$exclude_fmts
     $showfiltfmts scale | awk -F '[ \r]' '/^OUTPUT/{ print $3 }' | sort | comm -23 - $exclude_fmts >$out_fmts
 
-    pix_fmts=$($showfiltfmts $filter | awk -F '[ \r]' '/^INPUT/{ print $3 }' | sort | comm -12 - $out_fmts)
+    pix_fmts=$($showfiltfmts $filter $filter_args | awk -F '[ \r]' '/^INPUT/{ print $3 }' | sort | comm -12 - $out_fmts)
     for pix_fmt in $pix_fmts; do
         do_video_filter $pix_fmt "slicify=random,format=$pix_fmt,$filter=$filter_args" -pix_fmt $pix_fmt
     done