test: use headless chrome for dev

Author: yyx990803

Diff for: test/unit/karma.dev.config.js

@@ -2,7 +2,7 @@ var base = require('./karma.base.config.js')
 
 module.exports = function (config) {
   config.set(Object.assign(base, {
-    browsers: ['Chrome'],
+    browsers: ['ChromeHeadless'],
     reporters: ['progress'],
     plugins: base.plugins.concat([
       'karma-chrome-launcher'