Introduce and use a variable $output in the lavfi pixfmts test code.

Consistent with the lavfi pixdesc test code, and slightly improve
readability.

Originally committed as revision 24642 to svn://svn.ffmpeg.org/ffmpeg/trunk

tests/lavfi-regression.sh

@@ -78,7 +78,8 @@ if [ -n "$do_pixfmts_be" ] || [ -n "$do_pixfmts_le" ]; then
         pix_fmts=$(get_common_elements "$in_pix_fmts" "$scale_out_pix_fmts")
 
         for pix_fmt in $pix_fmts; do
-            do_video_encoding "${pix_fmt}-${filter}.nut" "" \
+            output=${pix_fmt}-${filter}.nut
+            do_video_encoding $output "" \
                 "-vf slicify=random,format=$pix_fmt,$filter_args -vcodec rawvideo -pix_fmt $pix_fmt"
         done
     done