selftests: add test for specifying 500 lower layers

Verify that we can actually specify 500 lower layers and fail at the
501st one.

Link: https://lore.kernel.org/r/20241015-leiht-filmabend-a86eed4ff304@brauner


Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>