Flaky test TestGdmModel

[adombeck]

I'm seeing both

TestGdmModel/Authenticated_with_preset_PAM_user_updated_and_server_side_broker_and_authMode_selection`

and

TestGdmModel/Authenticated_after_auth_selection_stage_from_client_after_client_side_broker_and_auth_mode_selection_if_there_is_only_one_auth_mode

fail very frequently in the autopkgtest CI jobs:

• https://github.com/ubuntu/authd/actions/runs/17977117879/job/51134399827?pr=1087#step:4:34788
• https://github.com/ubuntu/authd/actions/runs/18007390035/job/51231956033?pr=1091#step:4:34823
• https://github.com/ubuntu/authd/actions/runs/18007390035/job/51231956030?pr=1091#step:4:34360
• https://github.com/ubuntu/authd/actions/runs/18007390035/job/51231956009?pr=1091#step:4:34089