Change test_concurrent_futures.ThreadPoolShutdownTest

Adjust the shutdown test so that, after submitting three jobs
to the executor, the test checks for less than three threads,
instead of looking for exactly three threads.

If idle threads are being recycled properly, then we should have
less than three threads.

Author: iUnknwn

Lib/test/test_concurrent_futures.py

@@ -349,7 +349,7 @@ def test_threads_terminate(self):
         self.executor.submit(mul, 21, 2)
         self.executor.submit(mul, 6, 7)
         self.executor.submit(mul, 3, 14)
-        self.assertEqual(len(self.executor._threads), 3)
+        self.assertTrue(len(self.executor._threads) < 3)
         self.executor.shutdown()
         for t in self.executor._threads:
             t.join()