add selftest for statmount/listmount

Initial selftest for the new statmount() and listmount() syscalls.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Link: https://lore.kernel.org/r/20231213161104.403171-1-mszeredi@redhat.com
Signed-off-by: Christian Brauner <brauner@kernel.org>