tests: Create default RemoteRealms in populate_db.

This default setup will be more realistic, matching the ordinary
conditions for a modern server.
Especially needed as we add bouncer code that will expect to have
RemoteRealm entries for realm_uuid values for which it receives
requests.

zerver/lib/test_classes.py

@@ -2385,14 +2385,13 @@ def get_topic_messages(user_profile: UserProfile, stream: Stream, topic_name: st
 class BouncerTestCase(ZulipTestCase):
     @override
     def setUp(self) -> None:
+        # Set a deterministic uuid for convenience.
         self.server_uuid = "6cde5f7a-1f7e-4978-9716-49f69ebfc9fe"
-        self.server = RemoteZulipServer(
-            uuid=self.server_uuid,
-            api_key="magic_secret_api_key",
-            hostname="demo.example.com",
-            last_updated=timezone_now(),
-        )
+        self.server = RemoteZulipServer.objects.all().latest("id")
+
+        self.server.uuid = self.server_uuid
         self.server.save()
+
         super().setUp()
 
     @override