Fix gio/tests/socket-listener

g_socket_listener_add_address() is synchronous; all of the events will
have been emitted before it returns and it doesn't queue any sources.
The test was unintentionally depending on the fact that
g_main_context_iterate(NULL, TRUE) would return anyway (at least the
first time it was called), but that's no longer true after e4ee307.

https://bugzilla.gnome.org/show_bug.cgi?id=768968

gio/tests/socket-listener.c

@@ -74,12 +74,9 @@ test_event_signal (void)
                                  NULL,
                                  &error);
   g_assert_no_error (error);
+  g_assert_true (success);
+
   g_object_unref (saddr);
-
-  do
-    g_main_context_iteration (NULL, TRUE);
-  while (!success);
-
   g_object_unref (listener);
 }