I have been thinking about this more and I would sacrifice features for implementation simplicity. In the case of jest, it seems like it has to reinvent parts of webpack, under the assumption that your testing system should help you with mocking and stubbing.
With ReScript and some form of dependency injection, I think developers wouldn’t need to use mocking. At that point, recreating webpack mechanisms no longer justified. You could implement your testing system as something that hooks into the final phases of webpack.
Some people have been pursuing this with vitejs: https://github.com/zigomir/vite-test. I hope more projects like this emerge. Then, you would have one centralized config for your dev server and test system.