Spawning and killing browser and dev server in parallel?

[JoshuaKGoldberg]

The setup and teardown async logics run in series now. Since #66 is a little cleaner to read, code snippets from there:

// setup
browser = await puppeteer.launch(config.launch)
// (couple of other lines)
if (config.server) await setupServer(config.server)

// teardown
await teardownServer()
await browser.close()

These could be done in parallel for speed:

// setup
const tasks = [puppeteer.launch(config.launch)]
if (config.server) tasks.push(setupServer(config.server))
[browser] = await Promise.all(tasks)

// setup
await Promise.all([
    teardownServer(),
    browser.close(),
])

[gregberge]

Good idea! Todo after #67 will be merged.