Forcing the output even when a test passes is interesting on my box.
When ZEO is involved, a single ClientThread tends to run all the way to
completion before another ClientThread manages to sneak in -- apart from
their first transactions, it's as if we ran one thread, then started the
next after the first finished (etc).  When ZEO isn't involved, threads
take turns.  When I see failures, they involve ZEO.  It's possible that
30 seconds just isn't long enough to wait on this box.