path: fixing a test that breaks on some machines.

A win32-only test was verifying that path.win32._makeLong('C:')
would return the current working directory.  This would only work if
current working directory was also on the C: device.  Fix is to grab
the device letter for current working directory, and pass that to
_makeLong().

PR-URL: https://github.com/nodejs/node/pull/6067


Reviewed-By: Trott - Rich Trott <rtrott@gmail.com>
Reviewed-By: Joao Reis <reis@janeasystems.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>