SF bug 613233: test_threadedtempfile hangs

A possibility to deadlock (on the hidden import lock) was created here
in 2.3, seemingly when tempfile.py started to call functions in
random.py.  The cure is "the usual":  don't spawn threads as a side
effect of importing, when the spawned threads themselves do imports
(directly or indirectly), and the code that spawned the threads is
waiting for the threads to finish (they can't finish, because they're
waiting for the import lock the spawner still holds).  Worming around
this is why the "test_main" mechanism was introduced in regrest, so
it's a straightforward fix.

NOT a bugfix candidate; the problem was introduced in 2.3.