whoami: add test to improve coverage

* tests/whoami: add windows test for issue #4614

* tests/whoami issue #4614: add all platform test

tests/by-util/test_whoami.rs

@@ -50,3 +50,8 @@ fn test_normal_compare_env() {
         println!("test skipped:");
     }
 }
+
+#[test]
+fn test_succeeds_on_all_platforms() {
+    new_ucmd!().succeeds().no_stderr();
+}