Do not use new G_APPLICATION_DEFAULT_FLAGS enum value

The merge request !5235 switched tests and examples from
G_APPLICATION_FLAGS_NONE, which has been deprecated in GLib 2.74, to
G_APPLICATION_DEFAULT_FLAGS. Sadly, it was done unconditionally, which
means we'd have to bump the required version of GLib.

To avoid that, let's just use the numeric value of the enum member.

tests/testlist4.c

@@ -202,7 +202,6 @@ test_app_new (void)
 
   test_app = g_object_new (test_app_get_type (),
                            "application-id", "org.gtk.testlist4",
-                           "flags", G_APPLICATION_DEFAULT_FLAGS,
                            NULL);
 
   return test_app;