test: verify restore to specific backup in backup / restore tests

[mmols]

Summary

This PR improves the backup / restore tests by properly checking that we can restore to a specific backup, rather than the only backup. When you solely pass in set to pgBackRest with a backup label, the default behavior of pgBackRest is to restore to that backup, and then recover to the latest point in the archive. Setting type to immediate tells Postgres to only recover until the database becomes consistent. The previous test did not differentiate between these two scenarios.

Checklist

• Tests added or updated (unit and/or e2e, as needed)
• Documentation updated (if needed)
• Issue is linked (branch name or URL in PR description)
• Changelog entry added for user-facing behavior changes
• Breaking changes (if any) are clearly called out in the PR description

[rshoemaker]

approved.