[libc++][test] Change IsSmallObject's calculation for std::any's small object buffer

`sizeof(std::any) - sizeof(void*)` is correct for both libc++ and the MSVC standard library.

Differential Revision: https://reviews.llvm.org/D68756

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@374407 91177308-0d34-0410-b5e6-96231b3b80d8