Learning from Open Source Colocate test files

Issue #160

• Why you should co-locate your Xcode tests
• Files

https://github.com/kickstarter/ios-oss/blob/master/Kickstarter-iOS/ViewModels/RootViewModel.swift
https://github.com/kickstarter/ios-oss/blob/master/Kickstarter-iOS/ViewModels/RootViewModelTests.swift