Fix 003_extrafiles.pl test for the Windows

Fix 003_extrafiles.pl test for the Windows

File::Find converts backslashes to slashes in the newer Perl versions.
See: https://github.com/Perl/perl5/commit/414f14df98cb1c9a20f92c5c54948b67c09f072d

So, do the same conversion for Windows before comparing paths. To
support all Perl versions, always convert them on Windows regardless of
the Perl's version.

Author: Nazir Bilal Yavuz <byavuz81@gmail.com>

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