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.

'verify' iff it's used by a test module...

and replaces them with a new API verify(). As a result the regression
suite will also perform its tests in optimization mode.

Written by Marc-Andre Lemburg. Copyright assigned to Guido van Rossum.

(testall.py): added test_thread to the list of regression tests.