Add negative tests

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 2d751d6..144c70d 100644
--- a/library/ssl_tls13_server.c
+++ b/library/ssl_tls13_server.c
@@ -412,8 +412,8 @@
         }
     }
 
-    MBEDTLS_SSL_DEBUG_MSG( 2, ( "ssl_tls13_pick_key_cert: "
-                                "no suitable signature algorithm found" ) );
+    MBEDTLS_SSL_DEBUG_MSG( 2, ( "ssl_tls13_pick_key_cert:"
+                                "no suitable certificate found" ) );
     return( -1 );
 }
 #endif /* MBEDTLS_X509_CRT_PARSE_C &&