[RFC] Only run a subset of testcases in separated processes

This is quite a crude implementation that works, so RFC.
I'm having trouble getting this to work - I've added the decorator to my tests, but it looks like this third level of `test` doesn't exist in the test object for me. What am I missing? This probably plays into the comment to look into a better way to do this though if it depends on tests being written in a specific way.