systemclock: Add test for gst_clock_get_resolution
systemclock: Add test for gst_clock_get_resolution

In a series of time measurements, the diff between now and previous
timestamps is either 0 or at least as long as get_resolution returned.

https://bugzilla.gnome.org/show_bug.cgi?id=758012

StreamUtils/gstreamer,jpxiong/gstreamer,jpxiong/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,jpxiong/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,jpxiong/gstreamer,centricular/gstreamer,centricular/gstreamer,centricular/gstreamer,centricular/gstreamer,StreamUtils/gstreamer