test_subs: Use HostRequestMock for request object.

I incorrectly removed this when simplifying
dbddbee5a115b9352862cb13d4c66820865c30b6; while that commit did not
require the hunk re-added here, the later commit
3be622ffa7 added a call that did require it.

zerver/tests/test_subs.py

@@ -64,6 +64,7 @@ from zerver.lib.streams import (
 )
 from zerver.lib.test_classes import ZulipTestCase
 from zerver.lib.test_helpers import (
+    HostRequestMock,
     cache_tries_captured,
     get_subscription,
     most_recent_message,
@@ -3512,6 +3513,7 @@ class SubscriptionRestApiTest(ZulipTestCase):
         user_profile = self.example_user("hamlet")
         user_profile.full_name = "Hamlet"
         user_profile.save()
+        request = HostRequestMock(user_profile=user_profile)
 
         def thunk1() -> HttpResponse:
             user_profile.full_name = "Should not be committed"