How do I write a unit test to check that my code throws a CapabilityDisabledError when writing to GAE's datastore during an outage?

gistfile1.py

# coding=utf-8
import unittest
from google.appengine.api import users, capabilities
from google.appengine.ext import testbed
from google.appengine.runtime.apiproxy_errors import CapabilityDisabledError
from models.user import User


class TestDatastoreDown(unittest.TestCase):
    def setUp(self):
        self.tb = testbed.Testbed()
        self.tb.activate()
        self.tb.setup_env(USER_EMAIL='[email protected]', USER_ID='1', USER_IS_ADMIN='0', overwrite=True)

        self.tb.init_capability_stub()
        self.tb.init_memcache_stub()
        self.tb.init_datastore_v3_stub()
        self.stub = self.tb.get_stub('capability_service')
        self.stub.SetPackageEnabled('datastore_v3', False)

        self.guser = users.get_current_user()

    def tearDown(self):
        self.tb.deactivate()

    def test_stuff(self):

        # this test always passes, making you think that the capability for datastore is temporarily turned off
        self.assertFalse(capabilities.CapabilitySet('datastore_v3').is_enabled())

        # but then trying to actually write to the datastore actually works just fine and no exception is raised
        self.assertRaises(CapabilityDisabledError, lambda: User(
            guser=self.guser, user_handle='foo').put())

Stack trace of error:

Failure
Traceback (most recent call last):
File "/tmp/test_datastore_offline.py", line 28, in test_stuff
self.assertRaises(CapabilityDisabledError, lambda: User(
AssertionError: CapabilityDisabledError not raised