first cut at pulling test results out of the vm

When running in Jenkins it's rather convenient to be able to utilise minitest's junit output. This adds some config to allow one to do so.

I don't think this should be merged as is - I dislike having to specify the result for each platform, and it doesn't yet cope with multiple platforms all that well - it'll overwrite rather than having discreet directories per platform. But I want to start a discussion as to how best to proceed.