Tests calling the Sails binary directly fail on Windows

[sgress454]

Originally from #3466 (comment)

Calls like exec(sailsBin + ' new ' + appName, ...) where sailsBin is './bin/sails.js' fail on Windows due to the use of the forward slash, as well as the inconsistent association of .js files with the Node executable (the shebang a the top of sails.js obviously doesn't work on Windows).

Probably best to replace exec with a fork call, since it uses the Node binary implicitly. Then we don't have to worry about the .js association. The path problem can be fixed by just using path.sep or path.resolve().