rtpulpfec: stop and start the harness when setting error-after
gstreamer!55 makes some changes to how the `error-after` counter works which breaks this test. This change makes the test not rely on the ability to alter `error-after` at runtime and explicitly stops and starts the harness before pushing data. An alternative would be to add another argument to `harness_rtpulpfecdec` to set `error-after` on construction but that's slightly more long-winded. so I went for this approach instead. Fixes #532, even though that's already closed.