This ticket is split off from WT-12357. That ticket only deals with test/csuite/incr_backup. This ticket is for test/format work.
Currently test/format takes an incremental backup into a directory always called backup on each iteration. Then it runs recovery on that directory to verify the backup contents.

This ticket adds retaining a copy of the last N incremental backups. This could be done in a way similar to timestamp_abort which retains 5 backups.

We want this to be able to run a post-execution script/analysis on two backups.