test_push_notifications: Fix leak that can leak to test flakes.

While reviewing #11012, I discovered a nondeterministic result for
test_signup, which I tracked down to specifically this triple of tests
failing when run in this order:

test-backend GCMSuccessTest \
  zerver.tests.test_push_notifications.TestAPNs.test_get_apns_client \
  zerver.tests.test_signup.LoginTest.test_register

with a query count mismatch like this:

expected length: 73
actual length: 79

Comparing the list of queries, it's clear that test_register was
seeing `push_notifications_enabled()` returning True in this test order.

It's not clear why GCMSuccessTest was required here (it was!), but
further debugging determined the problem was that
`test_get_apns_client` left the _apns_client initialization system in
a state where get_apns_client would return a non-None value, resulting
in push_notifications_enabled() returning True for future tests.

The immediate fix is to just reset the `_apns_client` and
`_apns_client_initializedstate` state properly after the test runs;
but arguably we should do a larger refactor to make this less
fragile.

zerver/tests/test_push_notifications.py

@@ -707,12 +707,20 @@ class TestAPNs(PushNotificationTest):
             self.user_profile.id, devices, payload_data)
 
     def test_get_apns_client(self) -> None:
+        """This test is pretty hacky, and needs to carefully reset the state
+        it modifies in order to avoid leaking state that can lead to
+        nondeterministic results for other tests.
+        """
         import zerver.lib.push_notifications
         zerver.lib.push_notifications._apns_client_initialized = False
         with self.settings(APNS_CERT_FILE='/foo.pem'), \
                 mock.patch('apns2.client.APNsClient') as mock_client:
             client = get_apns_client()
             self.assertEqual(mock_client.return_value, client)
+        # Reset the values set by `get_apns_client` so that we don't
+        # leak changes to the rest of the world.
+        zerver.lib.push_notifications._apns_client_initialized = False
+        zerver.lib.push_notifications._apns_client = None
 
     def test_not_configured(self) -> None:
         with mock.patch('zerver.lib.push_notifications.get_apns_client') as mock_get, \