Testing: Add a way to simulate slow network or CPU in e2e tests

[gziolo]

Related #15538, #14986.

We constantly encounter issues with tests which fail randomly. It's more noticeable on Travis. It's usually difficult to reproduce those failures locally but it turns out that we can simulate slow network or CPU with Puppeteer API. I opened an issue argos-ci/jest-puppeteer#241 to see whether it'd be a good idea to include such debugging tools into jest-puppeteer. If it gets rejected we should integrate it into our e2e test setup ourselves.

Full proposal:
argos-ci/jest-puppeteer#241.

Example of debugging code used with other PRs in the past:

	const client = await page.target().createCDPSession();
	await client.send( 'Network.emulateNetworkConditions', {
		// Whether chrome should simulate
		// the absence of connectivity
		offline: false,
		// Simulated download speed (bytes/s)
		downloadThroughput: 1.5 * 1024 * 1024 / 8,
		// Simulated upload speed (bytes/s)
		uploadThroughput: 750 * 1024 / 8,
		// Simulated latency (ms)
		latency: 40,
	} );
	await client.send( 'Emulation.setCPUThrottlingRate', { rate: 4 } );