test_presence: Change assert_max_length to assert_length.

zerver/tests/test_presence.py

@@ -23,6 +23,7 @@ from zerver.models import (
     UserActivity,
     UserProfile,
     UserPresence,
+    flush_per_request_caches,
 )
 
 import datetime
@@ -44,10 +45,11 @@ class ActivityTest(ZulipTestCase):
                 count=count,
                 last_visit=last_visit
             )
+        flush_per_request_caches()
         with queries_captured() as queries:
             self.client_get('/activity')
 
-        self.assert_max_length(queries, 13)
+        self.assert_length(queries, 2)
 
 class TestClientModel(ZulipTestCase):
     def test_client_stringification(self):