Turn off format testing for relaxed isolation levels

WT-5994 Turn off format testing for relaxed isolation levels (part 2)

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• Closed

WT-5957 __rec_update_save, 35: onpage_upd != NULL || supd_restore: truncate not checking conflict in read committed

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• Closed

WT-5657 test_wt2323_join_visibility: relaxed isolation level failure

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• Closed

WT-6008 Turn off format stress testing for column-store

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• Closed