[v6.d REVIEW] Improve IO.mode tests
[v6.d REVIEW] Improve IO.mode tests

- Use our standard temp file routines for temp files
- Shorten test descriptions
- Use more appropriate test routines
- Toss full-file Windows fudge. Any failures here will be dealt
    separately, as part of resolving #320

perl6/roast,dogbert17/roast,dogbert17/roast