Made test_idle_thread_reuse more reliable (#92)

Depending on machine load or other unknown influences
on the Linux scheduler, this would often fail with

   File "test_futures.py", line 558, in test_idle_thread_reuse
     self.assertEqual(len(executor._threads), 1)
 AssertionError: 2 != 1
diff --git a/test_futures.py b/test_futures.py
index c2ae2d0..5ee4e71 100644
--- a/test_futures.py
+++ b/test_futures.py
@@ -577,7 +577,9 @@
     def test_idle_thread_reuse(self):
         executor = self.executor_type()
         executor.submit(mul, 21, 2).result()
+        time.sleep(0.001)
         executor.submit(mul, 6, 7).result()
+        time.sleep(0.001)
         executor.submit(mul, 3, 14).result()
         self.assertEqual(len(executor._threads), 1)
         executor.shutdown(wait=True)