This extension have been moved into the Roc extension monorepo here.
This change has been made to make it easier to work with the extensions overall and release new versions quickly. Please use this repo for reporting issues and open pull requests.
The code for roc-plugin-test-mocha-karma-webpack can be found here.
roc-plugin-test-mocha-karma-webpack 
Adds a way to easily test Webpack projects for the web in Roc
To be used together with Roc.