title: Running polymer-3.x test in headless browser
body: <p>I want to run tests without the browser pop-up.Installed <a href="https://www.npmjs.com/package/wct-headless" rel="nofollow noreferrer">wct-headless</a> and configured <strong>wct.config.json</strong> as shown</p>
  
  <pre><code>{
    "plugins": {
      "local": {
        "disabled": true
      },
      "headless": {
        "browsers": [
          "chrome",
          "firefox"
        ],
        "browsersOptions": {
          "chrome": [
            "headless",
            "disable-gpu",
            "no-sandbox"
          ],
          "firefox": [
            "--headless"
          ]
        }
      },
      "sauce": false,
      "xunit-reporter": {
        "output": "testfile.xml"
      }
    }
  }
  </code></pre>
  
  <p>While executing <code>polymer test</code>, it gives following <strong>error</strong></p>
  
  <blockquote>
    <p>Could not find WCT plugin named "headless"</p>
  </blockquote>
  
  <p><strong>package.json</strong></p>
  
  <pre><code>"dependencies": {
    "@polymer/polymer": "^3.0.0"
  },
  "devDependencies": {
    "@polymer/iron-demo-helpers": "^3.0.0-pre.19",
    "@webcomponents/webcomponentsjs": "^2.0.0",
    "wct-browser-legacy": "^1.0.0",
    "wct-headless": "^2.2.2",
    "web-component-tester": "^6.8.0",
    "wct-xunit-reporter": "1.0.2"
  }
  </code></pre>
  
  <p>How should i configure wct.conf.json ??</p>