rtpulpfec: stop and start the harness when setting error-after
gstreamer!55 (merged) 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 have been to add another argument to
harness_rtpulpfecdec to set
error-after on construction but that's
slightly more long-winded.
Fixes #532 (closed), even though that's already closed.