fate: add test for the thumbnail video filter.

tests/lavfi-regression.sh

@@ -43,6 +43,7 @@ do_lavfi "scale500"           "scale=500:500"
 do_lavfi "select"             "select=not(eq(mod(n\,2)\,0)+eq(mod(n\,3)\,0))"
 do_lavfi "setdar"             "setdar=16/9"
 do_lavfi "setsar"             "setsar=16/11"
+do_lavfi "thumbnail"          "thumbnail=10"
 do_lavfi "vflip"              "vflip"
 do_lavfi "vflip_crop"         "vflip,crop=iw-100:ih-100:100:100"
 do_lavfi "vflip_vflip"        "vflip,vflip"

tests/ref/lavfi/thumbnail

+thumbnail           9ad9ccc0b87ec4d1da26e4231fe00a1e