tests: enhance the gstcaps test_features to also test gst_caps_set_features()
tests: enhance the gstcaps test_features to also test gst_caps_set_features()

Compliments my previous patch for gst_caps_set_features, which would
previously assert and leak the old GstCapsFeatures if the caps already
had a GstCapsFeatures and you were trying to replace it with a new one.

shelsonjava/gstreamer,jpakkane/gstreamer,jpxiong/gstreamer,centricular/gstreamer,jpakkane/gstreamer,centricular/gstreamer,mparis/gstreamer,surround-io/gstreamer,surround-io/gstreamer,jpakkane/gstreamer,centricular/gstreamer,krichter722/gstreamer,StreamUtils/gstreamer,mparis/gstreamer,jpxiong/gstreamer,shelsonjava/gstreamer,mparis/gstreamer,shelsonjava/gstreamer,jpxiong/gstreamer,Lachann/gstreamer,jpakkane/gstreamer,surround-io/gstreamer,Lachann/gstreamer,krichter722/gstreamer,shelsonjava/gstreamer,Lachann/gstreamer,jpxiong/gstreamer,centricular/gstreamer,krichter722/gstreamer,krichter722/gstreamer,shelsonjava/gstreamer,jpakkane/gstreamer,mparis/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,krichter722/gstreamer,StreamUtils/gstreamer,surround-io/gstreamer,Lachann/gstreamer,Lachann/gstreamer,StreamUtils/gstreamer,mparis/gstreamer,surround-io/gstreamer