bug#32246: 27.0.50; files-tests failure on Cygwin

[Eli Zaretskii]

> From: Michael Albinus <michael.albinus@gmx.de>
> Date: Fri, 27 Jul 2018 21:30:24 +0200
> Cc: 32246@debbugs.gnu.org
> 
> There's the other proposal from Eli, use the root of the volume. This
> would work also, but I don't know how to find the root of a volume
> easily.

Ken's patch actually does something very similar, so unless Cygwin
makes it easy to find the volume root, I think Ken's patch is OK.