stbt.py: Add `videorate` to the video saving pipeline

Running a stbt test with `--save-video` without `videorate` in the video
saving pipeline makes the test fail with "general stream error:
streaming task paused, reason not-negotiated" when the video stream
restarts after the set-top-box is rebooted. `videorate` element adjusts
inconsistent timestamps of video frames, resolving the issue.

For more details see https://jira.youview.co.uk/browse/TESTAUT-1561.