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

diff --git a/tests/oauth2/test_credentials.py b/tests/oauth2/test_credentials.py
index fa86a16..b117ad4 100644
--- a/tests/oauth2/test_credentials.py
+++ b/tests/oauth2/test_credentials.py
@@ -53,7 +53,8 @@
 
     @mock.patch('google.oauth2._client.refresh_grant', autospec=True)
     @mock.patch(
-        'google.auth._helpers.utcnow', return_value=datetime.datetime.min)
+        'google.auth._helpers.utcnow',
+        return_value=datetime.datetime.min + _helpers.CLOCK_SKEW)
     def test_refresh_success(self, now_mock, refresh_grant_mock):
         token = 'token'
         expiry = _helpers.utcnow() + datetime.timedelta(seconds=500)