Agreed that a check like this should go into a function. I think `#on_windows?` in `Nanoc::TestHelpers` would be a good place to put this. Another benefit of putting it in a function is that you can [memoize](https://github.com/nanoc/nanoc/blob/master/lib/nanoc/base/memoization.rb) it.

I personally don’t really care about testing for `msdos` or `djgpp` (nobody uses these anyway).