Follow-ups from #924: worker watch between gates, waitForChildExit deadline #929

Follow-ups from the review of #924 (non-blocking), tracked here as one issue.

1. No worker-level test covers the between-gates half of the worker owner watch (the stdin and interval checks while a worker is not parked at a gate); only the parked case, which is test: worker children in two more test files should exit with the test process #712's case, is tested.
2. waitForChildExit resolves undefined at its deadline, so a red that reads "expected 0, received undefined" cannot distinguish a timeout from a non-zero exit. Reject with a message naming the deadline instead.

Refs #712, #924.