work around backslash issues with Win32 paths in taint test

work around backslash issues with Win32 paths in taint test

this hopefully addresses cpantesters failure reports, e.g.:
http://www.cpantesters.org/cpan/report/0c37f68f-6d57-1014-a7ec-a16b5be8bcae
http://www.cpantesters.org/cpan/report/f7b20263-6da8-1014-ab0b-a16a5be8bcae
http://www.cpantesters.org/cpan/report/0c37f68f-6d57-1014-a7ec-a16b5be8bcae
http://www.cpantesters.org/cpan/report/5486eef0-6c1a-1014-b801-4f72db05e747
http://www.cpantesters.org/cpan/report/bd043c85-6bf8-1014-9fed-19f2c69cc7be
http://www.cpantesters.org/cpan/report/c8a7c865-6d66-1014-b964-310d2901c822
http://www.cpantesters.org/cpan/report/2fbf0826-1fac-11e3-8778-8bb49a6ffe4e
http://www.cpantesters.org/cpan/report/a7b22cee-1f90-11e3-8778-8bb49a6ffe4e
http://www.cpantesters.org/cpan/report/1add08c4-6bf9-1014-a3b9-f229e79c0688
http://www.cpantesters.org/cpan/report/809a1d6d-6cca-1014-9c8f-bf33ec4105d1
http://www.cpantesters.org/cpan/report/cf8e9f51-6c0f-1014-b1c8-f0a75dd0450a
http://www.cpantesters.org/cpan/report/ca4b52de-6bf3-1014-810b-761a736704e8
http://www.cpantesters.org/cpan/report/77c604d8-6cdc-1014-ad93-a84555012b15