This is part of what's needed to build without warnings on mingw64:
it was warning about the cast from void* to long that happened in
the places we were using test_{n,}eq on pointers.
The alternative here would have been to broaden tt_int_op to accept
a long long or an intptr_t, but that's less correct (since pointers
aren't integers), and would hurt the portability of tinytest a
little.
Fixes part of 7260.
