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, legacy/trac#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