[TIMOB-15983] Fold Titanium Diagnostic Test into CLI

https://jira.appcelerator.org/browse/TIMOB-15983
Suppose `config.get('cli.httpProxyServer')` return `http://localhost:8080`, now the command that gets executed is:

~~~
curlhttp://localhost:8080 -d "un=unit_tests@aptana.com&pw=unittestSSSSSSSS&mid=studio" https://api.appcelerator.net/p/v1/sso-login -sS -f
~~~

The problem here is there is no command called `curlhttp://localhost:8080`, so these curl calls should not work when a `cli.httpProxyServer` is defined.

As far as whether to run curl tests or not, I would recommend you do this:

~~~
appc.subprocess.findExecutable('curl' + (process.platform === 'win32' ? '.exe' : ''), function (err, executable) {
    if (err) {
        // skip curl tests
    } else {
        appc.subprocess.run(executable, [ /* args */ ], function (code, out, err) {
            // curl test passes if `code` is falsey and out is JSON parsed and successful
        });
    }
});
~~~