I really don't want this to be a bash file. Ruby can start up child processes just as well, and it works on Windows and is comprehensible.

The UI for running this script is also very painful. It shouldn't need to be run from `test/threadsafety`, and running it with no arguments or with `--help` or `-h` should print useful help output rather than crashing. It would also be nice to merge it into `multi_threaded_runner.rb`; just have it run itself as a subprocess if the user passes some sort of `--fresh-runtime` flag.