Variable FPS tests

[scotts]

Creating a new PR based off of #669. I can't push to that PR, so we need a new one.

This PR adds a new video which has variable FPS, commits some reference frames from it, and then uses it in two tests. Notably we only test with exact seeking mode. These tests would fail in approximate mode because we would get the wrong frames.

[scotts]

Drive by. I realized we had this logic twice: here and again at the end of the script. We only want it once. The dupe was introduced back in #448.

[scotts]

Status: the current video in the PR is failing on CUDA with errors similar to what we fixed with #448. I want to re-create the video, but I am unable to. The comment in the script uses the command:

ffmpeg -f lavfi -i testsrc=duration=10:size=128x128:rate=30 -vf "setpts=PTS*if(gt(N\,30)\,1.5\,1)" -c:v libx264 -bf 0 var_fps_video.mp4

But I don't have libx264 in my environment. I do have libopenh264, but when I use that to encode the video, I see the warnings:

[libopenh264 @ 0x47a3c0] [OpenH264] this = 0x0x644690, Warning:layerId(0) doesn't support profile(578), change to UNSPECIFIC profile
[libopenh264 @ 0x47a3c0] [OpenH264] this = 0x0x644690, Warning:bEnableFrameSkip = 0,bitrate can't be controlled for RC_QUALITY_MODE,RC_BITRATE_MODE and RC_TIMESTAMP_MODE without enabling skip frame.
[libopenh264 @ 0x47a3c0] [OpenH264] this = 0x0x644690, Warning:Change QP Range from(0,51) to (12,42)

And the resulting video does not have a variable frame rate. Shelving this for now until we can reliably recreate a variable framer rate video.