Fix some Windows tests

[ArtskydJ]

A bunch of the tests were failing on Windows because of the path separator.

This PR fixes that issue.

At least one of the tests still fail for me, but I don't have access to that private git repo Rich-Harris/degit-test-repo-private...

[bernardoadc]

I think you could have simply used path.normalize(file) with .map instead of replace and reduce

files = files.map(path.normalize)