aggregator: Fix leak in unit test

GST_PAD_PROBE_HANDLED means that we should've unreffed the probe data,
it was handled by us in one way or another.
diff --git a/tests/check/libs/aggregator.c b/tests/check/libs/aggregator.c
index 877c4fb..5f6ae2e 100644
--- a/tests/check/libs/aggregator.c
+++ b/tests/check/libs/aggregator.c
@@ -342,6 +342,8 @@
       g_atomic_int_inc (&test->flush_stop_events);
   }
 
+  gst_mini_object_unref (info->data);
+
   return GST_PAD_PROBE_HANDLED;
 }