to guess /which/ line the spawned thread is in at the time
sys._current_frames() is called:  we know it finished
enter_g.set(), but can't know whether the instruction
counter has advanced to the following leave_g.wait().
The latter is overwhelming most likely, but not guaranteed,
and I see that the "x86 Ubuntu dapper (icc) trunk" buildbot
found it on the other line once.  Changed the test so it
passes in either case.