And use a simple GStreamer pipeline as testsrcbin with GstTranscoder
doesn't let us easily set the framerate of the source and we end up
having videorate dropping frames leading to the rendered file having
an unprecise duration.
This should fix races with `check.gst-editing-services.pythontests.pyunittest.python.test_assets.TestTimeline.test_reload_asset`
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1130>