Test case fails on cygwin - lack of wide path support

On cygwin, boost::filesystem uses wide character paths.
The mapped_file implementation in iostreams only supports this on Windows proper.
In cygwin it falls back to open() and there is no conversion code from the wide string that allows it to work. Since the built-in path object supports wide and not wide, mapped_file currently cannot file any file if the path is wide because the path string is empty.

https://github.com/boostorg/iostreams/blob/develop/src/mapped_file.cpp#L264

I disabled one of the mapped file tests only for CYGWIN to work around this. I was going to hook that code into boost::locale to leverage from_utf, but that creates a dependency that isn't currently there so I decided to open an issue for it instead.