bpo-38614: Use test.support.LOOPBACK_TIMEOUT constant (GH-17554)

Replace hardcoded timeout constants in tests with LOOPBACK_TIMEOUT of
test.support, so it's easier to ajdust this timeout for all tests at
once.
diff --git a/Lib/test/test_imaplib.py b/Lib/test/test_imaplib.py
index 03cffbe..846392e 100644
--- a/Lib/test/test_imaplib.py
+++ b/Lib/test/test_imaplib.py
@@ -109,7 +109,7 @@
 
 
 class SimpleIMAPHandler(socketserver.StreamRequestHandler):
-    timeout = 1
+    timeout = support.LOOPBACK_TIMEOUT
     continuation = None
     capabilities = ''