16795: Fix false-positive bug detection.

16795: Fix false-positive bug detection.

The dispatcher simulation test occasionally fails with this error when
a crunch-run stub is killed, and the same container is rescheduled on
the same node before the first attempt's startup-phase sleep expires:

    bugf("[test] StubDriver bug or caller bug: pid %d exiting, running[%s]==%d", pid, uuid, svm.running[uuid])
... Error: [test] StubDriver bug or caller bug: pid 9 exiting, running[zzzzz-dz642-000000000000184]==0

Arvados-DCO-1.1-Signed-off-by: Tom Clegg <tom@tomclegg.ca>