Skip y4m test on non-seekable input

[fhvwy]

On a non-seekable input such as a pipe this check skips the first eight bytes which silently corrupts the stream.

This fixes the use-case of piping input from another program because some preprocessing or conversion is needed, such as with ffmpeg:

 ffmpeg -i input.mp4 -vf scale=1280:720,format=yuv422p10 -f rawvideo - | bin/oapv_app_enc -i /dev/stdin -w 1280 -h 720 --input-csp 2 -d 10 -z 60 --profile 422-10 --band 3 -o out.apv

[tuzm24]

Could you add DCO?

• https://github.com/AcademySoftwareFoundation/openapv/pull/79/checks?check_run_id=41825788261