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_imaplib.py b/Lib/test/test_imaplib.py
index 846392e..795276e 100644
--- a/Lib/test/test_imaplib.py
+++ b/Lib/test/test_imaplib.py
@@ -238,7 +238,7 @@
         # cleanup the server
         self.server.shutdown()
         self.server.server_close()
-        support.join_thread(self.thread, 3.0)
+        support.join_thread(self.thread)
         # Explicitly clear the attribute to prevent dangling thread
         self.thread = None