Added test-specific cache directories
Added test-specific cache directories

This commit should fix issue #161. Test-specific cache directories
should make filenames unique within a test, avoiding the Windows issue
where moving files cannot overwrite existing files.

stiphyMT/plantcv,stiphyMT/plantcv,danforthcenter/plantcv,jshoyer/plantcv,danforthcenter/plantcv,AntonSax/plantcv,danforthcenter/plantcv,stiphyMT/plantcv,jshoyer/plantcv,AntonSax/plantcv