title: Running only single changed test file using Karma with Webpack
body: <p>I've got a working Karma setup, using Webpack. Whenever I modify either a test file or any of its dependencies, <strong>all</strong> of the tests run. This is all good, but <strong>I'd really like it to run only the minimum number of tests</strong>. If I change a test file, it should run only that test file. If I change a source file, it should run whatever test files have that source file as a dependency (via <code>require</code>).</p>
  
  <p>Here's the relevant portion of my <code>karma.config.js</code>:</p>
  
  <pre><code>files: [
    {
      pattern: '**/*.test.js',
      watched: true,
      included: true,
      served: true
    },
  ],
  
  preprocessors: {
    '**/*.test.js': ['webpack']
  },
  
  webpack: {
    resolve: {
      root: path.join(__dirname, '..', 'webpack'),
      extensions: ['', '.js', '.jsx'],
    },
  
    module: {
      loaders: [
        {
          test: /.jsx?$/,
          loader: 'babel-loader',
          exclude: [/node_modules/],
          query: {
            presets: ['es2015', 'react']
          }
        },
      ]
    },
  },
  </code></pre>
  
  <p>The complete <code>karma.config.js</code> file is <a href="https://gist.github.com/fweep/edd7ada15f43d648feb8e1f16d899366" rel="nofollow noreferrer">in a gist</a>.</p>
  
  <p>I'm familiar with solutions mentioned in <a href="https://stackoverflow.com/questions/26603402/how-do-i-run-only-certain-tests-in-karma">other questions</a>, involving <code>process.env.npm_config_single_file</code>, but that's not what I'm trying to do. </p>
  
  <p>Based on my config above, I'm expecting it to handle each file as a separate webpack, which is also implied by the documentation 
  at the top of <a href="https://github.com/webpack/karma-webpack" rel="nofollow noreferrer">webpack/karma-webpack</a>. Instead, what I seem to get is a single webpack with two chunks, per the log:</p>
  
  <pre><code>                      Asset     Size  Chunks             Chunk Names
  employee_list_entry.test.js  1.07 MB       0  [emitted]  employee_list_entry.test.js
  employer_list_entry.test.js  1.07 MB       1  [emitted]  employer_list_entry.test.js
  chunk    {0} employee_list_entry.test.js (employee_list_entry.test.js) 1.01 MB
  </code></pre>
  
  <p>It always rebuilds both chunks even when I only save one test file.</p>