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/[email protected]/ https://patchwork.ozlabs.org/project/ltp/patch/[email protected]/#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.