Add test for filesystem extended attributes

[pevik]

It'd be good to have a simple test that sets up extended attributes and expects a file to be accessible or not.

For more info see:
https://lore.kernel.org/ltp/aOY8SAGEklIur9eW@yuki.lan/
https://patchwork.ozlabs.org/project/ltp/patch/20251007160516.145121-1-pvorel@suse.cz/#3593023

NOTE: We have few ACL related tests (None of them are part of any runtest file, therefore probably not run by anybody):

• tacl_xattr.sh => convert to C API, or delete, once we have already test covering basic ACL.
• Few python based nfsv4 tests => convert to nfs_lib.sh or delete.

[jswoodward]

I'm also interested in this as we observed a regression backported into the stable tree for xattrs over NFSv4.2:

https://lore.kernel.org/linux-nfs/528e7a88-9c63-43d4-8c67-50a36ceda8a7@gmail.com/

While this is about general xattrs I'm looking at how this could land in any NFS tests LTP contains.

[pevik]

Thanks for pointing the regression out. If you plan to write a new test / modify existing test and got stuck let us know.