Change the cpp test to be optional

Remove unnecessary defines from the test.
Test by defining TEST_CPP using makefiles or cmake.

programs/test/header_test.cpp

@@ -102,14 +102,6 @@
 
 #if defined(MBEDTLS_PLATFORM_C)
 #include "mbedtls/platform.h"
-#else
-#include <stdio.h>
-#include <stdlib.h>
-#define mbedtls_printf     printf
-#define mbedtls_snprintf   snprintf
-#define mbedtls_exit       exit
-#define MBEDTLS_EXIT_SUCCESS EXIT_SUCCESS
-#define MBEDTLS_EXIT_FAILURE EXIT_FAILURE
 #endif
 
 #if defined(MBEDTLS_MEMORY_BUFFER_ALLOC_C)