Investigate failures in multi-threaded indexing unit tests

[eddyashton]

Added in #4075, temporarily disabled in #4099.

Seems to only fail on IceLake executors.

[eddyashton]

Looks like this wasn't a deadlock, just some unlucky runs of the unit test hitting an internal watchdog timeout.

I'd added a naive exponential backoff to the unit tests (disruptive contention between callers is expected, and resolved by the app - this "app" does it with an increasing backoff), which only need a few unlucky collisions to hit the 10s timeout. I've replaced that with a smaller, simpler timeout. Just confirming that remains reliably under the 10s watchdog timer.