bug#56712: 29.0.50; x-dnd-tests-do-direct-save fails on Cygwin

[Eli Zaretskii]

> From: Po Lu <luangruo@yahoo.com>
> Cc: Ken Brown <kbrown@cornell.edu>,  56712@debbugs.gnu.org
> Date: Tue, 26 Jul 2022 10:10:49 +0800
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > FTR: if that test should ever run on native MS-Windows, we'd need that
> > third slash, because file://c:/foo/bar is not valid on Windows, you
> > must say file:///c:/foo/bar.
> >
> > So from my POV, it is best to make that code system-dependent, with
> > windows-nt using 3 slashes, not 2.
> 
> The code is specific to X -- as evidenced by the name.  Doesn't Cygwin
> (the only way I know of to get an X server on MS Windows) use Unix-style
> file names everywhere?

Yes, thus my "if this test should ever run" condition.

IOW, the code produces 3 slashes by concatenating 2 strings, but the
results are only correct on Posix filesystems.  So at least a comment
is in order there, because how long would it take, you think, for
someone to copy/paste that code into an entirely different context?