tests ~ disable failing 'test_hostname::test_hostname_ip' test on MacOS

- also adds FixME to prompt future correction

tests/by-util/test_hostname.rs

@@ -10,6 +10,8 @@ fn test_hostname() {
     assert!(ls_default_res.stdout.len() >= ls_domain_res.stdout.len());
 }
 
+// FixME: fails for "MacOS"
+#[cfg(not(target_os = "macos"))]
 #[test]
 fn test_hostname_ip() {
     let result = new_ucmd!().arg("-i").run();