fate: add test for the drawbox video filter.

tests/lavfi-regression.sh

@@ -33,6 +33,7 @@ do_lavfi "crop_scale"         "crop=iw-100:ih-100:100:100,scale=400:-1"
 do_lavfi "crop_scale_vflip"   "null,null,crop=iw-200:ih-200:200:200,crop=iw-20:ih-20:20:20,scale=200:200,scale=250:250,vflip,vflip,null,scale=200:200,crop=iw-100:ih-100:100:100,vflip,scale=200:200,null,vflip,crop=iw-100:ih-100:100:100,null"
 do_lavfi "crop_vflip"         "crop=iw-100:ih-100:100:100,vflip"
 do_lavfi "deshake"            "deshake"
+do_lavfi "drawbox"            "drawbox=224:24:88:72:#FF8010@0.5"
 do_lavfi "null"               "null"
 do_lavfi "scale200"           "scale=200:200"
 do_lavfi "scale500"           "scale=500:500"

tests/ref/lavfi/drawbox

+drawbox             2745ca2d47f0086fd1a09aa871e42122