Update display.h

display.h

@@ -22,6 +22,7 @@
 
 #define HELLO_WORLD    "Hello world!" /**< A hello world strgin. */
 #define GOODBYE_WORLD  "Goodbye cruel world" /**< A goodbye world string. */
+#define TEST_MESSAGE  "Test message" /**< Just a test message. */
 
 /** @brief   A silly function that wraps printf()
  * @details  This is simply to allow a demonstration of the doxygen markup for