Allow using PROGMEM outside of Arduino (fixes #1903)

Ported from 40daf56b5a

extras/tests/Misc/StringAdapters.cpp

@@ -2,17 +2,16 @@
 // Copyright © 2014-2023, Benoit BLANCHON
 // MIT License
 
-#define ARDUINOJSON_ENABLE_PROGMEM 1
-
-#include "custom_string.hpp"
-#include "progmem_emulation.hpp"
-#include "weird_strcmp.hpp"
+#include <Arduino.h>
 
 #include <ArduinoJson/Strings/IsString.hpp>
 #include <ArduinoJson/Strings/StringAdapters.hpp>
 
 #include <catch.hpp>
 
+#include "custom_string.hpp"
+#include "weird_strcmp.hpp"
+
 using namespace ArduinoJson::detail;
 
 TEST_CASE("ZeroTerminatedRamString") {

extras/tests/Misc/StringWriter.cpp

@@ -2,10 +2,13 @@
 // Copyright © 2014-2023, Benoit BLANCHON
 // MIT License
 
-#define ARDUINOJSON_ENABLE_ARDUINO_STRING 1
+#include <Arduino.h>
+
 #define ARDUINOJSON_STRING_BUFFER_SIZE 5
 #include <ArduinoJson.h>
+
 #include <catch.hpp>
+
 #include "custom_string.hpp"
 
 using namespace ArduinoJson::detail;