Allow using PROGMEM outside of Arduino (fixes #1903)

extras/tests/Helpers/Arduino.h

@@ -7,5 +7,7 @@
 #include "api/Print.h"
 #include "api/Stream.h"
 #include "api/String.h"
+#include "avr/pgmspace.h"
 
+#define ARDUINO
 #define ARDUINO_H_INCLUDED 1

extras/tests/Helpers/avr/pgmspace.h

@@ -2,6 +2,8 @@
 // Copyright © 2014-2023, Benoit BLANCHON
 // MIT License
 
+#pragma once
+
 #include <stdint.h>  // uint8_t
 
 #define PROGMEM