tests: rtpjitterbuffer: test cleanups/improvements
tests: rtpjitterbuffer: test cleanups/improvements

Use fail_unless and friends instead of g_assert
Factor seq-num checking out to separate function
Check more return-values from push and crank and others

https://bugzilla.gnome.org/show_bug.cgi?id=762254

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