Fixed error "No such file or directory WString.h" (fixes #1381)

extras/tests/Helpers/api/String.h

@@ -0,0 +1,45 @@
+// ArduinoJson - arduinojson.org
+// Copyright Benoit Blanchon 2014-2020
+// MIT License
+
+#pragma once
+
+#include <string>
+
+// Reproduces Arduino's String class
+class String {
+ public:
+  String() {}
+  explicit String(const char* s) : _str(s) {}
+
+  String& operator+=(const char* rhs) {
+    _str += rhs;
+    return *this;
+  }
+
+  size_t length() const {
+    return _str.size();
+  }
+
+  const char* c_str() const {
+    return _str.c_str();
+  }
+
+  bool operator==(const char* s) const {
+    return _str == s;
+  }
+
+  friend std::ostream& operator<<(std::ostream& lhs, const ::String& rhs) {
+    lhs << rhs._str;
+    return lhs;
+  }
+
+ private:
+  std::string _str;
+};
+
+class StringSumHelper;
+
+inline bool operator==(const std::string& lhs, const ::String& rhs) {
+  return lhs == rhs.c_str();
+}