Store the strings in the heap

extras/tests/JsonVariant/misc.cpp

@@ -10,6 +10,11 @@ TEST_CASE("VariantData") {
           true);
 }
 
+TEST_CASE("StringNode") {
+  REQUIRE(std::is_standard_layout<ArduinoJson::detail::StringNode>::value ==
+          true);
+}
+
 TEST_CASE("JsonVariant from JsonArray") {
   SECTION("JsonArray is null") {
     JsonArray arr;

extras/tests/JsonVariant/set.cpp

@@ -5,6 +5,8 @@
 #include <ArduinoJson.h>
 #include <catch.hpp>
 
+#include "Allocators.hpp"
+
 enum ErrorCode { ERROR_01 = 1, ERROR_10 = 10 };
 
 TEST_CASE("JsonVariant::set() when there is enough memory") {
@@ -128,7 +130,7 @@ TEST_CASE("JsonVariant::set() when there is enough memory") {
 }
 
 TEST_CASE("JsonVariant::set() with not enough memory") {
-  JsonDocument doc(1);
+  JsonDocument doc(1, FailingAllocator::instance());
 
   JsonVariant v = doc.to<JsonVariant>();