Improve test server error handling

test/support/runPlaywrightTests.js

@@ -10,7 +10,7 @@ const playwrightBrowsers = ['chromium', 'firefox', 'webkit'];
 const mochaServer = new MochaServer(/* playwrightTest: */ true);
 
 const runTests = async (browserType) => {
-  mochaServer.listen();
+  await mochaServer.listen();
   const browser = await browserType.launch();
   const context = await browser.newContext();
   const page = await context.newPage();

test/web_server.js

@@ -13,6 +13,8 @@ var express = require('express'),
  */
 
 class MochaServer {
+  servers = [];
+
   constructor(playwrightTest) {
     this.playwrightTest = playwrightTest;
   }
@@ -46,13 +48,28 @@ class MochaServer {
     app.use(express.static(__dirname));
 
     const port = process.env.PORT || 3000;
-    this.server = app.listen(port);
+    // Explicitly listen on the IPv4 and IPv6 loopback interfaces. If you don’t
+    // pass an address to `app.listen`, then it will bind in a manner that
+    // succeeds even if the IPv4 socket address is already in use, as long as
+    // the IPv6 socket address is free. This can lead to confusion.
+    await this.startServer(app, '127.0.0.1', port);
+    await this.startServer(app, '::1', port);
+
+    console.log(`Mocha test server listening on http://localhost:${port}/`);
+  }
 
-    console.log('Mocha test server listening on http://localhost:3000/');
+  async startServer(app, address, port) {
+    await new Promise((resolve, reject) => {
+      const server = app.listen(port, address, resolve);
+      this.servers.push(server);
+      server.once('error', reject);
+    });
   }
 
   close() {
-    this.server.close();
+    for (const server of this.servers) {
+      server.close();
+    }
   }
 }