Tests: add user-defined literal ""_s for std::string

extras/tests/Helpers/Literals.hpp

@@ -0,0 +1,12 @@
+// ArduinoJson - https://arduinojson.org
+// Copyright © 2014-2024, Benoit BLANCHON
+// MIT License
+
+#pragma once
+
+#include <string>
+
+// the space before _s is required by GCC 4.8
+inline std::string operator"" _s(const char* str, size_t len) {
+  return std::string(str, len);
+}