jest-dev-server requires a port option in order to launch command

[MannyC]

🐛 Bug Report

jest-dev-server requires a port option in order to launch command

To Reproduce

launch with a jest-puppeteer.config.js such as

module.exports = {
  server: {
    command: 'npm run start',
    launchTimeout: 30000
  }
}

Expected behavior

npm run start should be executed followed by a 30 second wait.

It's pretty easy to see that setup doesn't do anything if config.port is falsy. No error thrown, it just exits.

https://github.com/smooth-code/jest-puppeteer/blob/b92e76c52fa57b0767963f94fe68200ef798e1e8/packages/jest-dev-server/src/global.js#L90-L168

This also makes it impossible to get around #96 as the obvious way to do that is simply not supplying jest-dev-server with a port to check.