[libcxx] [test] Skip alloc counter checks for operations within the libc++ DLL

If libc++ is built as a DLL, calls to operator new within the DLL aren't
overridden if a user provides their own operator in calling code.
Therefore, the alloc counter doesn't pick up on allocations done within
std::string, so skip that check if running on windows. (Technically,
we could keep the checks if running on windows when not built as a DLL,
but trying to keep the conditionals simple.)

Differential Revision: https://reviews.llvm.org/D100219