unit-tests: remove unnecesary field from struct bbunit_listelem

unit-tests: remove unnecesary field from struct bbunit_listelem

In the initial submission struct bbunit_listelem was manipulated by
custom list functions implemented in bbunit.c. Since the tests are now
added to the list by llist_add_to_end(), which allocates the llist_t
objects behind the scenes, there's no need for the *next field.

function                                             old     new   delta
unit_main                                            142     141      -1
bbunit_strrstr_elem                                   24      16      -8
bbunit_obscure_weak_pass_elem                         24      16      -8
bbunit_obscure_strong_pass_elem                       24      16      -8
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/4 up/down: 0/-25)             Total: -25 bytes

Signed-off-by: Bartosz Golaszewski <bartekgola@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>