code: function saveScreenshot(driver, dir, testTitle) {
    const data = driver.takeScreenshot()
    const fileName = testTitle.replace(/[^a-zA-Z0-9]/g, '_')
      .concat('_')
      .concat(new Date().getTime())
      .concat('.png')
  
    const fullPath = path.resolve(dir, fileName)
    fs.writeFileSync(fullPath, data, 'base64')
    reportOutput({
      type: 'image',
      name: fileName,
      file: fullPath
    })
  }
docs: Saves current screenshot from driver.
  fileName is resolved from testTitle and saved under given directory.
  
  And the same is reported to test-result report with `image` type.
  
  Ex:
  if (this.currentTest.state !== 'failed') {
  wdUtil.saveScreenshot(driver, './', this.currentTest.title)
  }
  
  @param {Driver} driver sync'ed webdriver
  @param {String} dir directory under which screenshot will be stored
  @param {String} testTitle current test title for resolving fileName