[libcxx] [test] Use string().c_str() to convert a std::filesystem::path to a const char*

On Windows, path::value_type is wchar_t, so one can't pass the return
value of path::c_str() directly to std::remove().

This matches what was done for tests under std/input.output/filesystems
in 81db3c31 and
3784bdf2.

Differential Revision: https://reviews.llvm.org/D97458