function startMockServer(port, options = {}) {
  assert.ok(port);
  const server = http.Server((req, res) => {
    servers[port].polling = true;
    if (options.bad) {
      return res.end('can not be parsed with JSON.parse()');
    }
    return res.end(JSON.stringify({
      ok: true,
      result: [{
        update_id: 0,
        message: { text: 'test' },
      }],
    }));
  });
  return new Promise((resolve, reject) => {
    servers[port] = { server, polling: false };
    server.on('error', reject).listen(port, resolve);
  });
}
Start the static server, serving files in './data'
@param  {Number} port