A Test262 runner, and async generators as the biggest lever

The conformance runner lands one day after the pivot, and async generators are implemented as the largest single block of failing executions.

79d10480: "tools: add test262 conformance runner". dd820da7: "async generators (async function*) — the biggest conformance lever". From this day on, features were measured in executions passed, though it would take until 25 July for the runner to count them correctly.