bpo-38614: Use default join_thread() timeout in tests (GH-17559)

Tests no longer pass a timeout value to join_thread() of
test.support: use the default join_thread() timeout instead
(SHORT_TIMEOUT constant of test.support).
diff --git a/Lib/test/test_sched.py b/Lib/test/test_sched.py
index 3f84af2..f78944f 100644
--- a/Lib/test/test_sched.py
+++ b/Lib/test/test_sched.py
@@ -82,7 +82,7 @@
         self.assertEqual(q.get(timeout=TIMEOUT), 5)
         self.assertTrue(q.empty())
         timer.advance(1000)
-        support.join_thread(t, timeout=TIMEOUT)
+        support.join_thread(t)
         self.assertTrue(q.empty())
         self.assertEqual(timer.time(), 5)
 
@@ -137,7 +137,7 @@
         self.assertEqual(q.get(timeout=TIMEOUT), 4)
         self.assertTrue(q.empty())
         timer.advance(1000)
-        support.join_thread(t, timeout=TIMEOUT)
+        support.join_thread(t)
         self.assertTrue(q.empty())
         self.assertEqual(timer.time(), 4)