karma.conf.js

var grep = require('karma-webpack-grep')
var webpackConfig = require('./webpack.config')

module.exports = function (config) {
  webpackConfig.plugins = (webpackConfig.plugins || []).concat(grep({
    grep: config.grep,
    basePath: '.',
    testContext: './tests'
  }))

  config.set({
    // ...
  })
}