WT-18204 added follower-side reader threads to test/format that hold a snapshot transaction open and verify repeated scans and point lookups return identical rows, catching snapshot-isolation violations across disaggregated checkpoint pickups.

The verification only relies on snapshot isolation, nothing disaggregated-specific: any snapshot transaction must see stable keys and values for its whole lifetime. Generalize it into a background thread that runs in every test/format configuration (standalone, leader, follower), opening a snapshot transaction and re-reading the same rows for a few seconds before moving to a new position.

Notes:

• The worker threads' own repeatable-read checks (snap.c) cover reads within operational transactions; the background verifier adds long-lived read-only transactions that overlap checkpoints, eviction, compaction and (on followers) checkpoint pickups.
• Reads without a read timestamp exercise the strictest form of the invariant; timestamped variants could be added for configurations without timestamps.
• Suggested by Siddhartha Sankaran in the review of the WT-18204 stress test (PR #14320).