Windows: visual tests use dedicated directory

Make use of
- styles-dir
- images-dir
- output-dir
- unique-subdir

https://github.com/mapnik/mapnik/blob/eb34d19c9fdbdb4c96599a9524a512d8f137afbe/test/visual/run.cpp#L99-L102

to prevent race conditions when building x86 and x64 at the same time.
