karma start --grep tests/someTest.js

karma.conf.js

var webpack = require('webpack')
var webpackConfig = require('./webpack.config')

module.exports = function (config) {
  // prep your webpack config for karma
  delete webpackConfig.entry
  webpackConfig.devtool = 'cheap-inline-source-map'
  
  // karma --grep
  if (config.grep) {
    var pattern = config.grep
      .replace(/\.js$/, '')
      .replace(/Test$/, '')
    var testContext = './tests'
    webpackConfig.plugins = (webpackConfig.plugins || []).concat([
      new webpack.ContextReplacementPlugin(new RegExp(testContext), function (result) {
        if (result.request === testContext) {
          result.regExp = new RegExp('.*' + pattern + '.*Test$')
        }
      })
    ])
  }

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