Add unit test for missing SPS/PPS at beginning of segment

[j0sh]

If the force-idr option is omitted from the encoder AVOptions while using nvidia transcoding, there will be decode errors at the beginning of playback. However, disabling this option currently doesn't lead to any unit test failures. We really should check that SPS/PPS aways exists via unit tests as a way of preserving this behavior.

Ideally this option wouldn't be needed at all; ffmpeg itself should but smart enough to emit extradata for the first frame after a flush; that would mitigate the need for the force-idr option.

[yondonfu]

For reference, forced-idr is currently set here.