Add more tests on string control character escaping

Verify the escaping of:
- DEL: Ox7F
- C1 list: 0x80-0x9F

Author: PaulGrandperrin

json_tests/tests/test_json.rs

@@ -141,6 +141,9 @@ fn test_write_str() {
         ("foo", "\"foo\""),
         ("\u{0}", "\"\\u0000\""),
         ("\u{1F}", "\"\\u001F\""),
+        ("\u{7F}", "\"\\u007F\""),
+        ("\u{80}", "\"\\u0080\""),
+        ("\u{9F}", "\"\\u009F\""),
         ("\"", "\"\\\"\""),
         ("\\", "\"\\\\\""),
         ("\u{8}", "\"\\b\""),