appsrc: fix buffer leaks in tests

It internally uses gst_check_chain_func() so we
should call gst_check_drop_buffers() when tearing down tests to free
the buffers which have been exchanged through the pipeline.

https://bugzilla.gnome.org/show_bug.cgi?id=766226

tests/check/elements/appsrc.c

@@ -60,6 +60,7 @@ cleanup_appsrc (GstElement * appsrc)
 {
   GST_DEBUG ("cleanup_appsrc");
 
+  gst_check_drop_buffers ();
   gst_check_teardown_sink_pad (appsrc);
   gst_check_teardown_element (appsrc);
 }