[libcxx] [test] Fix filesystem_test_helper.h to compile for windows

Use .string() instead of .native() in places where we want to combine
paths with std::string.

Convert some methods to take a fs::path as parameter instead of
std::string, for cases where they are called with paths as
parameters (which can't be implicitly converted to std::string if
the path's string_type is wstring).

Differential Revision: https://reviews.llvm.org/D89530