Introduce MBEDTLS_PRIVATE macro.

Public structs members are considered private and should not
be used by users application.

MBEDTLS_PRIVATE(member) macro is intended to clearly indicate
which members are private.

Signed-off-by: Mateusz Starzyk <mateusz.starzyk@mobica.com>
diff --git a/tests/include/test/helpers.h b/tests/include/test/helpers.h
index 9bfe085..0f82a90 100644
--- a/tests/include/test/helpers.h
+++ b/tests/include/test/helpers.h
@@ -25,6 +25,8 @@
 #ifndef TEST_HELPERS_H
 #define TEST_HELPERS_H
 
+#define MBEDTLS_ALLOW_PRIVATE_ACCESS
+
 #if !defined(MBEDTLS_CONFIG_FILE)
 #include "mbedtls/config.h"
 #else