Added a provision to stop all threads before exiting from the test:

the change to regrtest.py to unload all newly imported modules did
something bad to the threads -- and I realized that they would never
stop!

Lib/test/test_fork1.py

@@ -18,8 +18,10 @@ NUM_THREADS = 4
 
 alive = {}
 
+stop = 0
+
 def f(id):
-    while 1:
+    while not stop:
         alive[id] = os.getpid()
         try:
             time.sleep(SHORTSLEEP)
@@ -53,5 +55,9 @@ def main():
         spid, status = os.waitpid(cpid, 0)
         assert spid == cpid
         assert status == 0, "cause = %d, exit = %d" % (status&0xff, status>>8)
+        global stop
+        # Tell threads to die
+        stop = 1
+        time.sleep(2*SHORTSLEEP) # Wait for threads to die
 
 main()