Testing what?

ffmpeg can do this:

ffmpeg -f rawvideo -s 1280x720 -i SomeLargeRandomFile blah.h264

Probably on linux you can use /dev/urandom for SomeLargeRandomFile. It's not working for me on windows but that's quite possibly because cygwin is janky and horrible.

I should add that for most testing purposes, large realistic video files are more useful than this sort of thing. Moreover, it only gives you random pixels, not random video encoder settings.