Add test without server certificate

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/library/ssl_tls13_server.c b/library/ssl_tls13_server.c
index 396fce6..ca19908 100644
--- a/library/ssl_tls13_server.c
+++ b/library/ssl_tls13_server.c
@@ -1450,6 +1450,7 @@
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
     if( mbedtls_ssl_own_cert( ssl ) == NULL )
     {
+        MBEDTLS_SSL_DEBUG_MSG( 2, ( "No certificate available." ) );
         MBEDTLS_SSL_PEND_FATAL_ALERT( MBEDTLS_SSL_ALERT_MSG_HANDSHAKE_FAILURE,
                                       MBEDTLS_ERR_SSL_HANDSHAKE_FAILURE);
         return( MBEDTLS_ERR_SSL_HANDSHAKE_FAILURE );