test: add coverage for app.listen() variants

test/app.listen.js

@@ -24,4 +24,32 @@ describe('app.listen()', function(){
       })
     })
   })
+  it('accepts port + hostname + backlog + callback', function (done) {
+    const app = express();
+    const server = app.listen(0, '127.0.0.1', 5, function () {
+      const { address, port } = server.address();
+      assert.strictEqual(address, '127.0.0.1');
+      assert(Number.isInteger(port) && port > 0);
+      // backlog isn’t directly inspectable, but if no error was thrown
+      // we know it was accepted.
+      server.close(done);
+    });
+  });
+  it('accepts just a callback (no args)', function (done) {
+    const app = express();
+    // same as app.listen(0, done)
+    const server = app.listen();
+    server.close(done);
+  });
+  it('server.address() gives a { address, port, family } object', function (done) {
+    const app = express();
+    const server = app.listen(0, () => {
+      const addr = server.address();
+      assert(addr && typeof addr === 'object');
+      assert.strictEqual(typeof addr.address, 'string');
+      assert(Number.isInteger(addr.port) && addr.port > 0);
+      assert(typeof addr.family === 'string');
+      server.close(done);
+    });
+  });
 })