The idea is to consume what will be made available after WT-16434: the oldest_timestamp field.

The application, here test/model, should set the oldest timestamp after restart if it was set previously to avoid errors when validating contents that depend on the oldest timestamp.

Upon restart, WT should set this new field internally and test/model should be able to read it inside kv_workload_runner_wt::wiredtiger_open_nolock().