Three times a diagnostic has not run because the thing before it threw: `must`
on a command that exits non-zero, and then a query that hung long enough to
take the harness's own timeout with it — which arrives as an error with no
evidence attached rather than as a failed assertion.
A thirty-second test costs fifteen minutes to re-run, so the evidence has to be
gathered whichever way it fails. One helper, used by every assertion here, and
the query is bounded on the machine rather than by the harness: a query that
hangs is a result, not an accident.