printf: output of double-quote should not be escaped

This is obtained by escaping the sequence `\"` as `"`.

tests/by-util/test_printf.rs

@@ -36,6 +36,11 @@ fn escaped_slash() {
         .stdout_only("hello\\ world");
 }
 
+#[test]
+fn unescaped_double_quote() {
+    new_ucmd!().args(&["\\\""]).succeeds().stdout_only("\"");
+}
+
 #[test]
 fn escaped_hex() {
     new_ucmd!().args(&["\\x41"]).succeeds().stdout_only("A");