Fix cmocka-related compiler warnings in ht_test

Fix cmocka-related compiler warnings in ht_test

tests/isc/ht_test.c triggers the following compiler warnings when built
against development versions of cmocka:

    In file included from ht_test.c:24:
    ht_test.c: In function ‘test_ht_full’:
    ht_test.c:68:45: warning: passing argument 2 of ‘_assert_ptr_equal’ makes pointer from integer without a cast [-Wint-conversion]
       68 |                 assert_ptr_equal((void *)i, (uintptr_t)f);
    /usr/include/cmocka.h:1513:56: note: in definition of macro ‘assert_ptr_equal’
     1513 | #define assert_ptr_equal(a, b) _assert_ptr_equal((a), (b), __FILE__, __LINE__)
          |                                                        ^
    /usr/include/cmocka.h:2907:36: note: expected ‘const void *’ but argument is of type ‘long unsigned int’
     2907 |                        const void *b,
          |                        ~~~~~~~~~~~~^
    ht_test.c:163:45: warning: passing argument 2 of ‘_assert_ptr_equal’ makes pointer from integer without a cast [-Wint-conversion]
      163 |                 assert_ptr_equal((void *)i, (uintptr_t)f);
    /usr/include/cmocka.h:1513:56: note: in definition of macro ‘assert_ptr_equal’
     1513 | #define assert_ptr_equal(a, b) _assert_ptr_equal((a), (b), __FILE__, __LINE__)
          |                                                        ^
    /usr/include/cmocka.h:2907:36: note: expected ‘const void *’ but argument is of type ‘long unsigned int’
     2907 |                        const void *b,
          |                        ~~~~~~~~~~~~^

These are caused by a change to the definitions of pointer assert
functions in cmocka's development branch [1].  Fix by casting the
affected variables to (void *) instead of (uintptr_t).

[1] https://git.cryptomilk.org/projects/cmocka.git/commit/?id=09621179af67535788a67957a910d9f17c975b45