test(stty): add test for invalid free word arguments

Add a new test case `invalid_free_word_fails_immediately` to verify that `stty` appropriately handles and rejects invalid arguments like 'foo' and 'bar', ensuring the command fails with an error message "invalid argument 'foo'". This improves test coverage for argument validation.

tests/by-util/test_stty.rs

@@ -147,6 +147,14 @@ fn invalid_setting() {
         .stderr_contains("invalid argument 'igpar'");
 }
 
+#[test]
+fn invalid_free_word_fails_immediately() {
+    new_ucmd!()
+        .args(&["foo", "bar"])
+        .fails()
+        .stderr_contains("invalid argument 'foo'");
+}
+
 #[test]
 fn invalid_baud_setting() {
     #[cfg(not(any(