VideoDevice: Correct videotestsrc and set "is-live" to true.

src/video-device.vala

@@ -134,9 +134,11 @@ namespace Queso {
         }
 
         public new Gst.Element create_element(string name) {
-            if (is_test)
-                return Gst.ElementFactory.make("videoaudiosrc", name);
+            if (!is_test)
                 return base.create_element(name);
+            var src = Gst.ElementFactory.make("videotestsrc", name);
+            src.set("is-live", true);
+            return src;
         }
     }
 }