[libc++][test] Portability fix for std::any tests

Ensure that `large_tracked_t` defined in `any_helpers.h` is in fact too large to fit in `std::any`'s small object buffer.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@374806 91177308-0d34-0410-b5e6-96231b3b80d8