Test: add unit test for rend_data_t create/free/dup/* functions.
We've recently added rend_data_service_create()
and rend_data_client_create()
but without tests. Also, #15881 (moved) changed rend_data_dup()
and rend_data_free
a bit so we badly need unit testing for those to avoid future issue we might have by adding stuff to rend_data_t