[libcxx] [test] Make filesystem tests not rely on libc++ internals

As part of https://reviews.llvm.org/D119036
(506cf6dc), `-DNOMINMAX` was
dropped from the Windows CI configurations, replaced with a
block with `_LIBCPP_PUSH_MACROS`, `#include <__undef_macros>`
and `_LIBCPP_POP_MACROS` (and
`ADDITIONAL_COMPILE_FLAGS: -DNOMINMAX` left in two tests).

However, this workaround breaks the running the libc++ tests
against a different C++ standard library than libc++, as those
macros and that header are libc++ internals.

Therefore, reinstate `-DNOMINMAX` for clang-cl configurations
and remove the libc++ specific bits in filesystem_test_helper.h.

Differential Revision: https://reviews.llvm.org/D120478