selftests/filesystems: add open() test for anonymous inodes

Test that anonymous inodes cannot be open()ed.

Link: https://lore.kernel.org/20250407-work-anon_inode-v1-9-53a44c20d44e@kernel.org


Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>