tests: make xattr test more robust

Currently the xattr test asserts that listxattr() on Cargo.toml returns
a length of 0.

This causes a spurious failure when Cargo.toml does have xattrs, and
this can happen in fairly normal situations like when a Linux
distribution enables selinux (as it does on an AlmaLinux 10 test system,
for example):

$ cat /etc/almalinux-release
AlmaLinux release 10.0 (Purple Lion)

$ getfattr -d -m - Cargo.toml
security.selinux=[...]

$ cargo test --features=fs,stdio xattr
[...]
thread 'xattr::xattr_basic' panicked at tests/fs/xattr.rs:88:5:
assertion `left == right` failed
  left: 17
 right: 0

This change fixes the failure by making the test not assume that the
file has no xattrs defined.

Given that, pretty much the only reasonable thing the test can assume
about a call to listxattr is that it should succeed for a file that is
known to exist. So this makes the test just assert that it succeeds.

Author: bertschingert

tests/fs/xattr.rs

@@ -85,8 +85,8 @@ fn xattr_basic() {
             .raw_os_error(),
         enodata
     );
-    assert_eq!(rustix::fs::listxattr("Cargo.toml", &mut empty).unwrap(), 0);
-    assert_eq!(rustix::fs::llistxattr("Cargo.toml", &mut empty).unwrap(), 0);
+    assert!(rustix::fs::listxattr("Cargo.toml", &mut empty).is_ok());
+    assert!(rustix::fs::llistxattr("Cargo.toml", &mut empty).is_ok());
     assert_eq!(
         rustix::fs::removexattr("Cargo.toml", "user.test")
             .unwrap_err()