Fix flaky UpdateAllMirrorWorker test

Related to: !180874 (merged)

The test was introduced in: !168349 (merged)

I haven't had a chance to test it but this looks suspicious: https://gitlab.com/gitlab-org/gitlab/-/blob/68cdfc24848e5872eeebc9b10b95d5438ac7b7eb/ee/app/workers/update_all_mirrors_worker.rb#L15