bpo-38614: Use test.support.INTERNET_TIMEOUT constant (GH-17565)

Replace hardcoded timeout constants in tests with INTERNET_TIMEOUT of
test.support, so it's easier to ajdust this timeout for all tests at
once.
diff --git a/Lib/test/test_urllibnet.py b/Lib/test/test_urllibnet.py
index 848ab84..422d529 100644
--- a/Lib/test/test_urllibnet.py
+++ b/Lib/test/test_urllibnet.py
@@ -16,10 +16,8 @@
 class URLTimeoutTest(unittest.TestCase):
     # XXX this test doesn't seem to test anything useful.
 
-    TIMEOUT = 30.0
-
     def setUp(self):
-        socket.setdefaulttimeout(self.TIMEOUT)
+        socket.setdefaulttimeout(support.INTERNET_TIMEOUT)
 
     def tearDown(self):
         socket.setdefaulttimeout(None)