[resource-timing] Add more fine-grained tests for connection timing

[noamr]

The following metrics in the resource-timing specs are measured while obtaining a connection:
connectStart, connectEnd, secureConnectStart, dnsLookupStart, dnsLookupEnd, nextHopProtocol.

To test them, there probably needs to be a mock web server that delays the different steps, to ensure that they're measured correctly.

See https://w3c.github.io/resource-timing/

[noamr]

CC @yoavweiss

[noamr]

web-platform-tests/rfcs#112 should address this.