bpo-38614: Use test.support.LONG_TIMEOUT constant (GH-17562)

Replace hardcoded timeout constants in tests with LONG_TIMEOUT of
test.support, so it's easier to ajdust this timeout for all tests at
once.

LONG_TIMEOUT is 5 minutes by default, but it can be longer depending
on --timeout command line option.

Lib/test/test_asyncio/test_sslproto.py

@@ -16,6 +16,7 @@ import asyncio
 from asyncio import log
 from asyncio import protocols
 from asyncio import sslproto
+from test import support
 from test.test_asyncio import utils as test_utils
 from test.test_asyncio import functional as func_tests
 
@@ -163,7 +164,7 @@ class SslProtoHandshakeTests(test_utils.TestCase):
 class BaseStartTLS(func_tests.FunctionalTestCaseMixin):
 
     PAYLOAD_SIZE = 1024 * 100
-    TIMEOUT = 60
+    TIMEOUT = support.LONG_TIMEOUT
 
     def new_loop(self):
         raise NotImplementedError