test: x509: add function for testing long serials

Signed-off-by: Valerio Setti <vsetti@baylibre.com>
diff --git a/tests/suites/test_suite_x509write.data b/tests/suites/test_suite_x509write.data
index aa8b3cd..4cff30d 100644
--- a/tests/suites/test_suite_x509write.data
+++ b/tests/suites/test_suite_x509write.data
@@ -132,3 +132,6 @@
 
 X509 String to Names #6 (Escape at end)
 mbedtls_x509_string_to_names:"C=NL, O=Offspark\":"":MBEDTLS_ERR_X509_INVALID_NAME
+
+Check max serial length
+x509_set_serial_check: