test(uname): Add a test for the -o/--operating-system flag

tests/test_uname.rs

@@ -15,3 +15,13 @@ fn test_uname_name() {
     let result = ucmd.arg("-n").run();
     assert!(result.success);
 }
+
+#[test]
+fn test_uname_kernel() {
+    let (_, mut ucmd) = at_and_ucmd!();
+
+    let result = ucmd.arg("-o").run();
+    assert!(result.success);
+    #[cfg(target_os = "linux")]
+    assert!(result.stdout.to_lowercase().contains("linux"));
+}