Currently test/format and incr_backup take an incremental backup into a backup directory on each iteration. Then both run recovery (with a copy) to verify the backup contents. If there is a failure both programs have the saved un-recovered backup directory.

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.