Start adding unit test for session serialisation

This initial test ensures that a load-save function is the identity. It is so
far incomplete in that it only tests sessions without tickets or certificate.
This will be improved in the next commits.
diff --git a/tests/suites/test_suite_ssl.data b/tests/suites/test_suite_ssl.data
index 72092cd..aec0e9c 100644
--- a/tests/suites/test_suite_ssl.data
+++ b/tests/suites/test_suite_ssl.data
@@ -8752,3 +8752,6 @@
 SSL TLS_PRF MBEDTLS_SSL_TLS_PRF_SHA256 SHA-256 not enabled
 depends_on:!MBEDTLS_SHA256_C
 ssl_tls_prf:MBEDTLS_SSL_TLS_PRF_SHA256:"1234567890abcdef1234567890abcdef1234567890abcdef1234567890abcdef1234567890abcdef1234567890abcdef":"1234567890abcdef1234567890abcdef1234567890abcdef1234567890abcdef1234567890abcdef1234567890abcdef1234567890abcdef1234567890abcdef":"test tls_prf label":"7f9998393198a02c8d731ccc2ef90b2c":MBEDTLS_ERR_SSL_FEATURE_UNAVAILABLE
+
+Session serialisation, load-save
+ssl_serialise_session_load_save: