GDBusConnection: Fix race in /gdbus/connection/life-cycle

GDBusConnection sets the closed flag in the worker thread, then adds an
idle callback (which refs the Connection) to signal this in the main
thread. The tests session_bus_down doesn't spin the mainloop, so the
"closed" signal will always fire if iterating the mainloop later (and
drops the ref when doing so). But _is_closed can return TRUE even before
signalling this, in which case the "closed" signal isn't fired and the
ref isn't dropped, causing the test to fail.

Instead simply always wait for the closed signal, which is a good thing
to check anyway and ensures the ref is closed.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=661896
Reviewed-by: Matthias Clasen <mclasen@redhat.com>

gio/tests/gdbus-connection.c

@@ -264,7 +264,6 @@ test_connection_life_cycle (void)
   g_assert (!g_dbus_connection_is_closed (c));
   g_dbus_connection_set_exit_on_close (c, FALSE);
   session_bus_down ();
-  if (!g_dbus_connection_is_closed (c))
   _g_assert_signal_received (c, "closed");
   g_assert (g_dbus_connection_is_closed (c));