Tests: use h264 encoder in mp4 tests with newer ffmpeg.

[defanator]

Proposed changes

Recent ffmpeg versions (e.g. 7.1.2 on Fedora) do not have libx264 encoder available, but h264 does the same thing. This PR proposes dynamic selection of an encoder based on what's available in the environment.

Checklist

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• I have read the contributing guidelines.
• I have signed the F5 Contributor License Agreement (CLA).
• If applicable, I have added tests that prove my fix is effective or that my feature works.
• If applicable, I have checked that any relevant tests pass after adding my changes.
• I have updated any relevant documentation (README.md and/or CHANGELOG.md).