add part of dlist test suit. includes tests for
- dlist_init
- dlist_new_list
- dlist_new_node
- dlist_print_node
- dlist_delete_node
- dlist_delete_list
- dlist_push_node
- dlist_append_node
- dlist_pop_node
- dlist_find_node
- dlist_find_index_of
- dlist_remove_node