Tests needed for crashes during backup restart

Summary
As a followup to WT-9926, we need some automated tests that ensure we have coverage of crashes during backup restart.

Motivation

We have been shown that starting up with a WiredTiger.backup file exercise code paths not normally taken, and the consequences of getting it wrong are pretty severe.

Acceptance Criteria (Definition of Done)

We need some automated testing that shows that crashes during a startup from a backup (or equivalently, starting up from a full directory copy made while the host system had a backup cursor open) can be full recovered, without loss of metadata or data.

One approach is to look at the various *_abort programs, and pick one to be used as a model, or maybe one of them could be extended.

Optional

Optionally, we could also use the test/syscall program to verify that the sequence of system calls we are making is sensible.