tests: rtpaux: use a dynamic pt in the test
tests: rtpaux: use a dynamic pt in the test

1) Tests that using dynamic PT instead of the default ones work
2) If we ever decide to change the codec here we don't need to
   worry about change the PT for the default one of the new codec
   in the test

https://bugzilla.gnome.org/show_bug.cgi?id=746445

Kurento/gst-plugins-good,hizukiayaka/gst-plugins-good,stfl/gst-plugins-good,sebras/gst-plugins-good,GStreamer/gst-plugins-good,sebras/gst-plugins-good,pexip/gst-plugins-good,hizukiayaka/gst-plugins-good,GrokImageCompression/gst-plugins-good,Kurento/gst-plugins-good,pexip/gst-plugins-good,rawoul/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,StreamUtils/gst-plugins-good,pexip/gst-plugins-good,Kurento/gst-plugins-good,StreamUtils/gst-plugins-good,GrokImageCompression/gst-plugins-good,hizukiayaka/gst-plugins-good,GrokImageCompression/gst-plugins-good,stfl/gst-plugins-good,pexip/gst-plugins-good,rawoul/gst-plugins-good,StreamUtils/gst-plugins-good,GStreamer/gst-plugins-good,hizukiayaka/gst-plugins-good,rawoul/gst-plugins-good,StreamUtils/gst-plugins-good,sebras/gst-plugins-good,rawoul/gst-plugins-good,stfl/gst-plugins-good,stfl/gst-plugins-good,Kurento/gst-plugins-good,Kurento/gst-plugins-good,GrokImageCompression/gst-plugins-good,sebras/gst-plugins-good