Add accessor helpers for mbedtls_test_info

Step one of being able to control access to mbedtls_test_info with
a mutex.

Signed-off-by: Paul Elliott <paul.elliott@arm.com>
diff --git a/programs/test/metatest.c b/programs/test/metatest.c
index b8dffa9..545129d 100644
--- a/programs/test/metatest.c
+++ b/programs/test/metatest.c
@@ -343,9 +343,11 @@
 #if defined(MBEDTLS_TEST_MUTEX_USAGE)
             mbedtls_test_mutex_usage_check();
 #endif
+            int result = (int) mbedtls_test_get_result();
+
             mbedtls_printf("Running metatest %s... done, result=%d\n",
-                           argv[1], (int) mbedtls_test_info.result);
-            mbedtls_exit(mbedtls_test_info.result == MBEDTLS_TEST_RESULT_SUCCESS ?
+                           argv[1], result);
+            mbedtls_exit(result == MBEDTLS_TEST_RESULT_SUCCESS ?
                          MBEDTLS_EXIT_SUCCESS :
                          MBEDTLS_EXIT_FAILURE);
         }