tests: Don't use caching session engine in tests.

Because we run tests in parallel, the caching session engine can lead
to nondeterministic failures.

zerver/tests/test_home.py

@@ -174,7 +174,7 @@ class HomeTest(ZulipTestCase):
         with queries_captured() as queries:
             result = self._get_home_page(stream='Denmark')
 
-        self.assert_length(queries, 39)
+        self.assert_length(queries, 41)
 
         html = result.content.decode('utf-8')
 
@@ -237,7 +237,7 @@ class HomeTest(ZulipTestCase):
         with queries_captured() as queries2:
             result = self._get_home_page()
 
-        self.assert_length(queries2, 33)
+        self.assert_length(queries2, 34)
 
         # Do a sanity check that our new streams were in the payload.
         html = result.content.decode('utf-8')

zerver/tests/test_presence.py

@@ -47,7 +47,7 @@ class ActivityTest(ZulipTestCase):
         with queries_captured() as queries:
             self.client_get('/activity')
 
-        self.assert_length(queries, 3)
+        self.assert_length(queries, 4)
 
 class TestClientModel(ZulipTestCase):
     def test_client_stringification(self):

zproject/test_settings.py

@@ -93,6 +93,9 @@ CACHES['database'] = {
     }
 }
 
+# Disable caching on sessions to make query counts consistent
+SESSION_ENGINE = "django.contrib.sessions.backends.db"
+
 # Use production config from Webpack in tests
 if CASPER_TESTS:
     WEBPACK_FILE = 'webpack-stats-production.json'