Add option to run tests from different interpreters in parallel

[asmeurer]

Now that I have a computer with eight cores, I'd like to be able to run the bot tests in parallel. Something like a command line option --cores N with would spawn up to N processes. Can this be done using the current subprocess framework, or would we need to use the multiprocessing module?

[bgee]

I've been experimenting with multiprocessing module and the concern I'm having is running multiple threads will cause the printing messed up. I tried using locks but had no luck. Maybe because we are running from cmd2?

Subprocess has a check_output method which can grap whatever being printed on stdout and convert it to a string which might be useful.