test: Fix compile-time warnings

test: Fix compile-time warnings

testsuite/gtk/notify.c: In function ‘test_type’:
testsuite/gtk/notify.c:434:14: warning: assignment to ‘GObject *’ {aka ‘struct _GObject *’} from incompatible pointer type ‘GtkSettings *’ {aka ‘struct _GtkSettings *’} [-Wincompatible-pointer-types]
     instance = g_object_ref (gtk_settings_get_default ());
              ^
testsuite/gtk/notify.c:443:16: warning: assignment to ‘GObject *’ {aka ‘struct _GObject *’} from incompatible pointer type ‘GdkWindow *’ {aka ‘struct _GdkWindow *’} [-Wincompatible-pointer-types]
       instance = g_object_ref (gdk_window_new (NULL, &attributes, 0));
                ^