[TIMOB-15983] Fold Titanium Diagnostic Test into CLI

https://jira.appcelerator.org/browse/TIMOB-15983
I tested with a local install of squid, though I was unsuccessful in configuring squid to actually block any traffic. So, `ti setup check` ran and reported that I had a proxy configured. But, the tests probably didn't sufficiently test such a configuration. 

As for running on Windows, see my comment on the ticket. I'd like guidance on the best way to handle the lack of cURL on Windows.