[CI] Fix Puppeteer launch timeout

Author: Kocal

test/functional.js

@@ -81,7 +81,7 @@ describe('Functional tests using webpack', function() {
     this.timeout(10000);
 
     before(function(done) {
-        puppeteer.launch().then(_browser => {
+        puppeteer.launch({ headless: true }).then(_browser => {
             browser = _browser;
             done();
         });