Canonicalize to \ instead on Windows

Previously https://github.com/martine/ninja/commit/65151e7eefce514febd0a9676c5363c6d4c54b66
CanonicalizePath was made to normalize both slash styles to /.

This had some undesirable side-effects due to other tools expecting
\ in string searches. Going the other way should fix #843.

It's a bit uglier in the test code, but it's probably better
behaviour to make ninja be more Windows-like on Windows anyway.