test(pr1053): use concrete server group ids

PR #1053 changes server-group cache behavior. Configure the shared OpenStack
client mock to return real string IDs for created server groups so the
disk-backed cache can serialize nodegroup test data instead of trying to pickle
a MagicMock id.

Signed-off-by: Rico Lin <rico@vexxhost.com>
Assisted-By: ChatGPT <noreply@openai.com>

Author: ricolin

magnum_cluster_api/tests/conftest.py

@@ -106,6 +106,9 @@ def mock_osc(session_mocker, image):
 
     # Others
     mock_clients.url_for.return_value = "http://fake_url"
+    mock_nova_client = mock_clients.nova.return_value
+    mock_nova_client.server_groups.list.return_value = []
+    mock_nova_client.server_groups.create.return_value.id = uuidutils.generate_uuid()
 
     return mock_clients