populate_db: Set wildcard_mention_policy to show warning in test_suite.

We set wildcard_mention_policy in the test database so that we can
avoid future changes in mention puppeteer tests, as the default
membership of streams in the Zulip development organization is large
enough to prevent random users from using wildcard mentions.

zilencer/management/commands/populate_db.py

@@ -270,6 +270,12 @@ class Command(BaseCommand):
                     string_id="lear", name="Lear & Co.", emails_restricted_to_domains=False,
                     invite_required=False, org_type=Realm.CORPORATE)
 
+                # Default to allowing all members to send mentions in
+                # large streams for the test suite to keep
+                # mention-related tests simple.
+                zulip_realm.wildcard_mention_policy = Realm.WILDCARD_MENTION_POLICY_MEMBERS
+                zulip_realm.save(update_fields=['wildcard_mention_policy'])
+
             # Create test Users (UserProfiles are automatically created,
             # as are subscriptions to the ability to receive personals).
             names = [