backport tim_one's checkin of

backport tim_one's checkin of
    revision 1.10 of test_thread.py
    revision 1.5 of test_threaded_import.py

SF bug #516372:  test_thread: unhandled exc. in thread
Fix exit races in test_thread.py and test_threaded_import.py.
I suspect the bug is provokable only under Linux (where child threads
seem to get lots of cycles before they get killed after the main thread
exits), or on multi-processor machines running other OSes.
Bugfix candidate.