Ensure that results files are unlikely to coincide on different branches

[daoudclarke]

At the moment, new results files are created with mkdtemp. It's possible (though perhaps unlikely?) that different users working on the same repo could create new results files with the same name. That would be annoying because someone would have to merge, probably by renaming one of the files.

It would be nice if we could be confident that this is impossible or very unlikely to happen.