Add a consistent 5 minute clock skew accomodation (#145)

diff --git a/tests/test_app_engine.py b/tests/test_app_engine.py
index d3a79d5..136a914 100644
--- a/tests/test_app_engine.py
+++ b/tests/test_app_engine.py
@@ -17,6 +17,7 @@
 import mock
 import pytest
 
+from google.auth import _helpers
 from google.auth import app_engine
 
 
@@ -111,7 +112,7 @@
 
     @mock.patch(
         'google.auth._helpers.utcnow',
-        return_value=datetime.datetime.min)
+        return_value=datetime.datetime.min + _helpers.CLOCK_SKEW)
     def test_refresh(self, now_mock, app_identity_mock):
         token = 'token'
         ttl = 100
@@ -124,7 +125,7 @@
             credentials.scopes, credentials._service_account_id)
         assert credentials.token == token
         assert credentials.expiry == (
-            datetime.datetime.min + datetime.timedelta(seconds=ttl))
+            now_mock() + datetime.timedelta(seconds=ttl))
         assert credentials.valid
         assert not credentials.expired