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/ssl_servers.py b/Lib/test/ssl_servers.py
index bfe533c..2e7e235 100644
--- a/Lib/test/ssl_servers.py
+++ b/Lib/test/ssl_servers.py
@@ -49,7 +49,7 @@
     server_version = "TestHTTPS/1.0"
     root = here
     # Avoid hanging when a request gets interrupted by the client
-    timeout = 5
+    timeout = support.LOOPBACK_TIMEOUT
 
     def translate_path(self, path):
         """Translate a /-separated PATH to the local filename syntax.