Create MockRedis class.

Redis is not available on Travis and creating a mock Redis server looks
easier than using `mock` module.

zerver/tests/test_push_notifications.py

@@ -1,4 +1,5 @@
 import mock
+from typing import Any
 
 from django.test import TestCase
 from django.conf import settings
@@ -6,14 +7,31 @@ from django.conf import settings
 from zerver.models import PushDeviceToken, UserProfile
 from zerver.models import get_user_profile_by_email
 from zerver.lib import push_notifications as apn
-from zerver.lib.redis_utils import get_redis_client
 
+class MockRedis(object):
+    data = {}  # type: Dict[str, Any]
+
+    def hgetall(self, key):
+        return self.data.get(key)
+
+    def exists(self, key):
+        return key in self.data
+
+    def hmset(self, key, data):
+        self.data[key] = data
+
+    def delete(self, key):
+        if self.exists(key):
+            del self.data[key]
+
+    def expire(self, *args, **kwargs):
+        pass
 
 class PushNotificationTest(TestCase):
     def setUp(self):
         email = 'hamlet@zulip.com'
-        self.redis_client = get_redis_client()
         apn.connection = apn.get_connection('fake-cert', 'fake-key')
+        self.redis_client = apn.redis_client = MockRedis()  # type: ignore
         apn.dbx_connection = apn.get_connection('fake-cert', 'fake-key')
         self.user_profile = get_user_profile_by_email(email)
         self.tokens = ['aaaa', 'bbbb']