Provide synchronization point once upgrades have been checked

As #6592 (closed) shows, at least our automated test suite needs to know when upgrades have been checked. A simple mechanism, similar to htpdate’s “done file”, would be good enough for the time being.

Feature Branch: feature/6508-incremental-upgrades-phase-four

Parent Task: #6508 (closed)

Related issues

Blocks #6592 (closed)

Original created by @intrigeri on 6593 (Redmine)

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