Wait for static Server to start in tests

This patch starts waiting for Server's listening event
before starting to run any tests.

test/StaticServer.js

@@ -25,6 +25,17 @@ const fulfillSymbol = Symbol('fullfill callback');
 const rejectSymbol = Symbol('reject callback');
 
 class StaticServer {
+  /**
+   * @param {string} dirPath
+   * @param {number} port
+   * @return {!StaticServer}
+   */
+  static async create(dirPath, port) {
+    let server = new StaticServer(dirPath, port);
+    await new Promise(x => server._server.once('listening', x));
+    return server;
+  }
+
   /**
    * @param {string} dirPath
    * @param {number} port

test/test.js

@@ -30,11 +30,11 @@ describe('Puppeteer', function() {
   let staticServer;
   let page;
 
-  beforeAll(function() {
+  beforeAll(SX(async function() {
     browser = new Browser({args: ['--no-sandbox']});
-    staticServer = new StaticServer(path.join(__dirname, 'assets'), PORT);
+    staticServer = await StaticServer.create(path.join(__dirname, 'assets'), PORT);
     GoldenUtils.removeOutputDir();
-  });
+  }));
 
   afterAll(function() {
     staticServer.stop();