cdgdec: fix test with gst master
cdgdec: fix test with gst master

Test was relying on 'blocksize' property on the source which can
only be used in push mode.

This change in baseparse broke it:
https://gitlab.freedesktop.org/gstreamer/gstreamer/-/commit/e906197c622725e48b6250a71a922d45b006fb14
so ensure we are actually in push mode by using pushfilesrc.

sdroege/rsplugin,sdroege/rsplugin