title: karma-eslint preprocessor not working
body: <p>I'm using <a href="https://www.npmjs.com/package/karma-eslint" rel="nofollow">karma-eslint</a> plugin. It looks very easy to use but for some reason, I don't see any errors or warnings and my tests are running smoothly even though I put some <em>eslint errors</em> </p>
  
  <p>here is my <code>karma.config.js</code> file:</p>
  
  <pre><code>module.exports = function (config) {
  config.set({
  
      browsers: [process.env.CONTINUOUS_INTEGRATION ? 'Firefox' : 'Chrome'],
  
      singleRun: true,
  
      frameworks: ['mocha'],
  
      files: [
          'tests.webpack.js'
      ],
  
      preprocessors: {
          'tests.webpack.js': ['webpack', 'sourcemap'],
          'src/**/*.jsx': ['coverage'],
          'test/**/*.js': ['eslint'],
      },
  
      eslint: {
          engine: {
              configFile: './.eslintrc',
              emitError: true,
              emitWarning: true
          }
      },
  
      reporters: ['progress', 'coverage'],
  
      coverageReporter: {
          /* coverage configurations */
      },
  
      webpack: {
          /* some webpack configurations */
      }
  </code></pre>
  
  <p>The violation I planted in the one of my <code>test.js</code> files - define a new variable but not using it (eslint rule: 'no-unused-vars')</p>
  
  <p>Please let me know if any further information is needed and I'll edit the post accordingly.</p>
  
  <p>Cheers!</p>