I would suggest taking a look at the Open source Composable Architecture. which has emphasis in testability. About the Swift Composable Architecture category
Also take a look at GitHub - OpenCombine/OpenCombine: Open source implementation of Apple's Combine framework for processing values over time. since the one that SwiftUI uses is only available in Apple platforms.
Finally you can look at GitHub - Cosmo/OpenSwiftUI: WIP — OpenSwiftUI is an OpenSource implementation of Apple's SwiftUI DSL. but I don't see much movement there. Good luck!