Resolve minor warnings in tests (reported by GCC with '-pedantic' option)

Resolve minor warnings in tests (reported by GCC with '-pedantic' option)

* tests/disclaim_test.c (pair_dct, pair_new): Explicitly cast
a pointer argument of GC_printf to void* to match %p format specifier.
* tests/test.c (cons, print_int_list, run_one_test, check_heap_stats):
Likewise.