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.
6 jobs for rtpulpfec-test-error-after in 28 minutes and 56 seconds (queued for 1 second)
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Status Job ID Name Coverage
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passed #66628
gst indent

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passed #66627
manifest

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  Build
passed #66629
build fedora x86_64

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  Test
passed #66630
check fedora

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  Full Builds
passed #66631
build android arm64 api28

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passed #66632
build cerbero fedora x86_64

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