async function startExpress() {
  return new Promise((resolve, reject) => {
    const p = spawn('node', ['express']);
    p.on('error', reject);
    p.stdout.on('data', data => {
      if (data.includes('Press Ctrl+C to quit.')) {
        resolve(p);
      }
    });
  });
}
Start the express server, and wait for it to start serving content before
resolving the promise.