test: Fix the structure_changed message test
test: Fix the structure_changed message test

The core changed to use sink pads for the structure changed
message instead of source pads. Might as well fix the test too.

GStreamer/gst-python,freedesktop-unofficial-mirror/gstreamer-sdk__gst-python,GStreamer/gst-python,lubosz/gst-python,pexip/gst-python,alessandrod/gst-python,lubosz/gst-python,alessandrod/gst-python,freedesktop-unofficial-mirror/gstreamer-sdk__gst-python,freedesktop-unofficial-mirror/gstreamer__gst-python,freedesktop-unofficial-mirror/gstreamer__gst-python,pexip/gst-python,pexip/gst-python,alessandrod/gst-python,freedesktop-unofficial-mirror/gstreamer-sdk__gst-python,GStreamer/gst-python,lubosz/gst-python,freedesktop-unofficial-mirror/gstreamer__gst-python,freedesktop-unofficial-mirror/gstreamer-sdk__gst-python