tests: fix up mpg123 test a little
tests: fix up mpg123 test a little

- dist input files
- fix sample leak
- simplify check for elements
- only run mpg123 test if mpg123 is available and selected
- fix build in uninstalled setup

https://bugzilla.gnome.org/show_bug.cgi?id=686595

pexip/gst-plugins-good,GrokImageCompression/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,StreamUtils/gst-plugins-good,StreamUtils/gst-plugins-good,pexip/gst-plugins-good,GrokImageCompression/gst-plugins-good,StreamUtils/gst-plugins-good,pexip/gst-plugins-good,StreamUtils/gst-plugins-good,pexip/gst-plugins-good,GrokImageCompression/gst-plugins-good,GStreamer/gst-plugins-good,GrokImageCompression/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good