lavf-regression: use -frames instead of -t for image tests

-t 0.5 is 12.5 frames at 25 fps, which may round to either 12 or 13 on
different platforms.

tests/lavf-regression.sh

@@ -30,7 +30,7 @@ do_image_formats()
     outfile="$datadir/images/$1/"
     mkdir -p "$outfile"
     file=${outfile}%02d.$1
-    run_avconv $DEC_OPTS -f image2 -vcodec pgmyuv -i $raw_src $2 $ENC_OPTS $3 -t 0.5 -y -qscale 10 $target_path/$file
+    run_avconv $DEC_OPTS -f image2 -vcodec pgmyuv -i $raw_src $2 $ENC_OPTS $3 -frames 12 -y -qscale 10 $target_path/$file
     do_md5sum ${outfile}02.$1
     do_avconv_crc $file $DEC_OPTS $3 -i $target_path/$file
     echo $(wc -c ${outfile}02.$1)