[libcxx] [test] Disable some allocation checks in class.path tests on windows

On windows, the path internal representation is wchar_t, and
input/output often goes through utf8 inbetween, which causes extra
allocations.

MS STL also fails a number of strict allocation checks, so this
shouldn't be a standards compliance issue.

Differential Revision: https://reviews.llvm.org/D98398