Just because project is small (initially) doesn't mean proper architecture is over-engineering. The fact that you want to write tests tells that your project isn't a completely trivial one-off hack.
You didn't mention which guiGUI-frameworkFramework you are using. With WPF MVVM (Model-View-ViewModel) is good and allows you to write tests for all the logic quite easily. With WinForms I've heard good things about MVP (Model-View-Presenter)