Adds spawnTest(file, { worker: true }), which runs the test file on a worker thread of the spawned child, plus a runtimeFeatures.worker flag to gate on. First consumer is the worker half of test_instance_data (#66).

Why an option on spawnTest rather than an in-process runInWorker: a native printf from an addon writes to fd 1 directly, bypassing a worker's piped JS-level stdout, so observing a secondary environment's native output means capturing the host process's stdout. Upstream wraps its worker in a child process for the same reason.

runtimeFeatures.worker is declared separately from spawn because the two are independent - a browser has workers but no subprocesses.

The worker inherits the child's execArgv, so harness globals reach it. Leaving the Worker without an error handler is deliberate: an unhandled worker error re-throws on the host thread, so a failing test file still exits non-zero with its stack on stderr. tests/harness/spawn-test-worker.js pins that, plus cwd.