[libc++] Remove incorrect assertion in the filesystem tests

Based on the current discussion in https://llvm.org/PR45307, it seems
that it's legitimate for `temp_directory_path()` to return a path with
a trailing slash. Since `p.parent_path()` will never contain a trailing
slash, comparing it to the result of `temp_directory_path()` will fail
depending on whether `temp_directory_path()` returns a trailing slash
or not.