lib/fs: Let xattr test avoid non-test attributes (fixes #8601) (#8628)
lib/fs: Let xattr test avoid non-test attributes (fixes #8601) (#8628)

SELinux for example adds security.* attributes by default that we are
not allowed to touch, which causes the test to fail.
syncthing/syncthing,syncthing/syncthing,syncthing/syncthing,syncthing/syncthing,syncthing/syncthing,syncthing/syncthing