skip test_iteration_errors if /root is missing (#14299)

# Description

`test_iteration_errors` no longer requires `/root` to exist:

```
failures:

---- test::test_iteration_errors stdout ----
thread 'test::test_iteration_errors' panicked at crates/nu-glob/src/li
b.rs:1151:13:
assertion failed: next.is_some()
```

`/root` is an optional home directory in the [File Hierarchy
Standard][1].

I encountered this while running the tests in a `guix shell` container,
which doesn't include a root user.

[1]: https://refspecs.linuxfoundation.org/FHS_3.0/fhs/ch03s14.html

# User-Facing Changes

None

crates/nu-glob/src/lib.rs

@@ -1144,8 +1144,17 @@ mod test {
         use std::io;
         let mut iter = glob("/root/*").unwrap();
 
-        // Skip test if running with permissions to read /root
-        if std::fs::read_dir("/root/").is_err() {
+        match std::fs::read_dir("/root/") {
+            // skip if running with permissions to read /root
+            Ok(_) => {}
+
+            // skip if /root doesn't exist
+            Err(err) if err.kind() == io::ErrorKind::NotFound => {
+                assert!(iter.count() == 0);
+            }
+
+            // should otherwise return a single match with permission error
+            Err(_) => {
                 // GlobErrors shouldn't halt iteration
                 let next = iter.next();
                 assert!(next.is_some());
@@ -1158,6 +1167,7 @@ mod test {
                 assert!(err.error().kind() == io::ErrorKind::PermissionDenied);
             }
         }
+    }
 
     #[test]
     fn test_absolute_pattern() {