ssl_helpers.c: add mbedtls_test prefix for tweak_tls13_certificate*

Signed-off-by: Yanray Wang <yanray.wang@arm.com>
diff --git a/tests/suites/test_suite_ssl.function b/tests/suites/test_suite_ssl.function
index c9178e3..33ab577 100644
--- a/tests/suites/test_suite_ssl.function
+++ b/tests/suites/test_suite_ssl.function
@@ -3358,7 +3358,7 @@
          * Tweak server Certificate message and parse it.
          */
 
-        ret = tweak_tls13_certificate_msg_vector_len(
+        ret = mbedtls_test_tweak_tls13_certificate_msg_vector_len(
             buf, &end, step, &expected_result, &expected_chk_buf_ptr_args);
 
         if (ret != 0) {