[toolset] Fix results.json contains only result of last test.

When I run tests, there is only one result in results.json.
It was because benchmarker makes child process and results updated in child process.
For what it's worth, this line creates potential to experience [UnboundLocalVariable](http://eli.thegreenplace.net/2011/05/15/understanding-unboundlocalerror-in-python/), which I just experienced by trying to add `log.debug("Before load %s" % str(results))`. Not sure if there's a way to ensure this doesn't happen to anyone else?